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164134 |[(] 


No, 6"fo//? 4l P Accession No. 

Author y\ 

Title -r -/ 
This book shquld b/returneci on or before the date lajt marked below. 



By RICHARD M. HOLMAN, Late Associate Pro- 
fessor of Botany in the College of Letters and Science 
of the University of California; and WILFRED W. 
BOBBINS, Professor of Botany in the College of Agri- 
culture of the University of California. Third Edition, 
392 pages. 6 by 9 &. 273 figures. Cloth. 


For Colleges and Universities. By the late RICHARD 
Edition, 664 pages. 6by9>. 482 figures. Cloth, 


By LEE BONAR, Associate Professor of Botany in 
the University of California, LUCILB ROUSH, Formerly 
of the Department of Biological Science, Mills College, 
and the late RICHAKD M. HOLMAN, Fourth Edition, 
110 pages. 6 by 9J4. Cloth. 





Professor of Botany in the College of Agriculture 
of the University of California 



Lecturer, Natural Science 
Loyola University , Chicago 

Sixth Printing 






All Rights Reserved 

^his book or any part thereof must not 
e reproduced in any form without 
ke written permission oj the publisher, 




It is the belief of the authors of this book that no subject con- 
tributes more, when properly taught, to the attainment of the 
cardinal principles of secondary education than does biological 
science. It is the main province of this text to lay a foundation 
of fundamental principles which will enable pupils to develop an 
understanding of the significance of plant life which is such an 
important part of their environment. Moreover, the work in 
botany should be made practical in the sense that it should supply 
a basis of fact necessary to an understanding of principles, so that 
the student can use them in developing within himself a degree of 
social, civic, ethical, and esthetic efficiency. 

A practical course in botany should aid in developing an 
appreciation of the possibilities of improvement of the home 
environment through putting into practice a knowledge of the 
principles of plant growth. Window-plant culture, landscaping 
of home grounds, vegetable and flower gardening not only con- 
tribute to the attractiveness of the home, but they also provide 
pleasant and profitable avocations as worthy use of leisure time. 
Also, knowledge of foods, bacteria, and the laws of sanitation, and 
the life out-of-doors occasioned by engaging in vocations and 
avocations along the lines of plant study and plant culture, both 
tend toward personal efficiency by making the person a healthier 

Certain aims and .objectives have been set up. The problems 
and exercises are such as to be a direct aid in the attainment of 
these aims and objectives. The teacher who administers the 
course should not only have in mind the general objectives, but 
he should also recognize locally adapted specific objectives which 
should aid in determining points of special emphasis. Local con- 
ditions which affect specific aims include: dominant interests of 
pupils, community interests and needs, and availability of local 
resources, as woods and streams, greenhouses, parks, farms, 



health laboratories, landscaped homes, milk-pasteurizing plants, 
canneries, and facilities for sewage disposal. 

The problem involves learning activities which when properly 
directed by the teacher and carried out by the pupil will lead to 
the development of significant biological ideas and to the acquisi- 
tion of the elements of scientific thinking. 

The course is organized as a series of problems and sub- 
problems. It is intended that each problem shall lead pupils 
inductively to an understanding of important generalizations. 
The introduction of the book is an over-view of the entire course, 
and the introduction of each unit is an over-view of that unit. 
Provision is made for meeting individual differences by including 
suggested activities and additional exercises and problems which 
may be done by pupils who are able to finish the required work 
ahead of the majority of the class. 

The arrangement of the units is logical as it stands, but it is 
not intended as the best order under all conditions. Teachers 
who prefer a seasonal arrangement will find it possible to change 
the order of presentation of units to suit their requirements. 
A course beginning at mid-year will require an arrangement of 
units different from that which is best for a course beginning in 
the fall. 

Teachers are referred to The Teaching of Biology, by William 
E. Cole, published by D. Appleton-Century Company, for helpful 
suggestions regarding points of view in biology, laboratories, 
equipment, bibliographies, and materials. 


September 9, 1935. JEROME ISENBARGER 





































































































PLANTS? 358 


INDEX 387 



Wherever man has gone on the earth he has found some kind 
of plant life. Expeditions to arctic and antarctic regions, to the 
tops of the highest mountains, into the sandy stretches of the 
driest deserts, to all parts of the world, have always revealed some 
form of plant life. In the ocean, in both fresh- and salt-water 
lakes, in ponds and streams, in drinking water, in the waters of 
hot springs, there is an abundance of plant life. Certain bacteria, 
fungi, and algae occur in countless numbers in the soil, without 
which organisms soil fertility would not be maintained; bacteria 
and fungi are ever-present in the air, stealing rides on floating dust 
particles; bacteria are also always present in the digestive tracts 
of all kinds of animals; in fact, both plants and animals serve as 
hosts to many different kinds of bacteria and fungi, some harm- 
less, or even beneficial, others disease-causing. Name five diseases 
of man caused by bacteria. 

In Schimper's Plant Geography, a monumental work of 839 
pages and 502 illustrations, published by the Clarendon Press, 
Oxford, England, we find the following: 

"As has been already shown, there is nowhere on earth a 
place too cold for plant life, and only a few spots of very limited 
area that are too hot. As regards light, there is no limitation; 
it is nowhere too dark, nowhere too bright to exclude plant life of 
some kind. In the depths of the ocean, where light is absolutely 
absent, the decaying corpses of animals are decomposed by bac- 
teria. ... In the well-known Guacharo Cave near Caribe in 
Venezuela we found the ground covered with patches of dense 
etiolated vegetation up to half a meter in height, which had sprung 
up from the dung of the Guacharo birds, the only inhabitants of 
the Cave." 


Further, Schimper says: " The perpetual snow and ice of the 
polar zone and of mountains, here and there, exhibit conspicuous 
coloring caused by microscopic algae. . . . The occurrence of algae 
associated with red snow has been demonstrated on the most 
distant points in the Arctic and Antarctic zones and on most 
mountains with perpetual snow, so that the phenomenon may be 
assumed to be of general distribution." 

Schimper quotes from Volkens' description of the vegetation 
on the highest peaks of Kilimanjara, a mountain of Africa which 
attains an elevation of 6010 meters. Volkens says, " Finally, at 
4500 meters we have reached the last outposts, all isolated plants, 
forming little cushions under the shelter of stones. . . . Beyond 
this, wherever the ground is dry, only lichens and mosses prevail." 

Thus we see that plants invade all sorts of environments, that 
is, they live under all kinds of conditions. To do so successfully, 
they must be fitted to the conditions of their surroundings. Most 
certainly land plants can not live in the water; and ordinary, thin- 
leaved water plants would soon succumb if transplanted to a dry 
hillside. Why? Plants accustomed to the shade of the forest 
floor do not survive if the forest is cut or burned over; and plants 
of the open can not thrive in the shade. The ability of plants 
to live in all sorts of environments is possible only because of their 
great variation in form and structure. For example, plants of the 
desert must possess those characteristics which enable them to 
survive where water is scarce. Some of these so-called drought- 
resistant characteristics are: water-storage tissue, greatly reduced 
leaf surface, and thick coverings on the leaves which cut down 
water loss. Many plants of high mountains are low and mat- 
forming, thus getting the warmth close to the soil and avoiding the 
greater loss of water which would occur if they grew several feet 

One has only to examine plants growing under varied condi- 
tions to learn how well they are fitted to their environment. How 
are lichens able to live upon bare rock surfaces? How can cer- 
tain orchids manage to survive on the branches of trees without 
any connection with the soil? How can cacti and many other 
plants live in the desert where the rainfall is but a few inches a 
year? What peculiarities do those plants have which can thrive 


in alkali flats? What structures enable the cypress to grow in 
swamps where water always covers the roots? Why is it possible 
to grow Durum wheat and certain sorghums on the dry plains of 
western United States without irrigation, whereas many other 
crops will not thrive there without irrigation? What character- 
istics would you expect those plants to have which can grow suc- 
cessfully in acid bogs? These are suggestive of the problems con- 
fronting the student who would know the relation of plants to 
their environment. 

Plants are living things. True it is they do not move about 
from place to place, as do most animals. But they manifest all 
the essential characters that we associate with livingness. Plants 
absorb materials from the outside world; they make food; they 
digest food; they respire; they make plant substances out of 
foods; they grow; they are sensitive to light, gravity, moisture, 
heat, and other environmental factors; they reproduce. Plants 
are indeed living things organisms. 

The fundamental organization and composition of plants and 
animals are quite similar. The unit of structure in both plants 
and animals is the cell, a microscopic sac containing the living 
stuff protoplasm. The cells are grouped to form tissues, such 
as absorbing tissue, conducting tissue, protective tissue, storage 
tissue, reproductive tissue, etc.; and the tissues are grouped to 
form organs, such as roots, leaves, stems, flowers, fruit, and seed. 

There are no chemical elements found in animals that do not 
occur in plants. This is a rather remarkable fact. In both, the 
principal elements which enter into the composition of the body 
are oxygen, potassium, magnesium, hydrogen, nitrogen, sulphur, 
phosphorus, and carbon. These are common elements which 
occur in the air and soil. The foods of plants and animals are the 
same. At first thought, we may question this statement. But 
the foods of both plants and animals are carbohydrates (sugars, 
starch, cellulose, etc.), fats, and proteins. What are the chemical 
characteristics of these three groups of foods? The processes of 
absorption, digestion, respiration, assimilation, growth, and repro- 
duction are essentially alike in plants and animals. This state- 
ment may also arouse doubt in the mind of the reader, but the 
discussion of these processes which will come in succeeding units 


will assist in removing this doubt. Of course, there are marked dif- 
ferences between plants and animals. For example, the great 
majority of plants are not able to move from place to place, 
whereas locomotion is characteristic of most animals; in the 
majority of plants each of the cells is surrounded by a relatively 
rigid wall, whereas the cells of animals are usually without such 
surrounding walls; from the very simple substances such as 
water, carbon dioxide, and mineral salts, obtained from the soil 
and air, most plants can build the foods necessary to nourish 
their bodies, whereas animals are unable to make their own foods; 
and the growth in length of most plants takes place at or near the 
ends of the organs, such growth generally continuing as long as the 
plant is alive, whereas in animals, growth is not usually restricted 
to the extremities, and ceases long before death. 

It is of interest to note that the very simplest plants and the 
very simplest animals, both groups of which are aquatic, have 
much more in common than do higher plants and higher animals. 
In fact, there is substantial evidence that the plant and animal 
kingdoms had a common ancestor; that, in the process of devel- 
opment of the races of plants and animals, the distinction between 
these two great groups of living things has become greater and 
greater. But they have retained, by virtue of their common 
ancestry, many essential features. In other words, life on this 
earth is much the same whether it expresses itself in plants or 

The present assemblage of plants in the world varies greatly in 
complexity. The ordinary trees, shrubs, and herbs are relatively 
complex plants, by which we mean that they possess many different 
organs and tissues for carrying on their life activities. For exam- 
ple, they have roots, stems, leaves, flowers, fruit, and seed. There 
are many plants, such. as the pond scums, seaweeds, bacteria, 
molds, mushrooms, etc., which do not have roots, stems, leaves, 
flowers, or seed. Such plants are simple in their bodily organiza- 
tion. Moreover, their methods of reproduction are not as com- 
plex and advanced as in seed plants. Then, there are such plants 
as mosses, liverworts, and ferns, which have roots or root-like 
structures, stems, and leaves, but no flowers or seeds. It is re- 
garded by students of plant life, who have carefully examined and 


compared the structure of a great many different kinds of plants 
and their methods of reproduction and coupled this with a study 
of fossil plants (Fig. 173), that during the past hundreds of thou- 
sands of years as life developed on the earth there has been great 
change in the nature of plants. There is reliable evidence that 
the first plants that appeared on the earth were water plants, 
similar in many particulars to our present-day pond scums; that 
from these primitive ancestors there developed more complex 
plants, such as liverworts and mosses; that, as the thousands of 
years passed by, there appeared ferns and their allies; and that 
from certain fern-like plants were developed our present-day seed 
plants. Seed plants are regarded as the most advanced and most 
complex of plants, just as man is considered the most advanced 
and complex of animals. 

The plant world as we see it today is not as it always was in 
the earth's history. For example, geological records show unmis- 
takably that the vegetative covering of a large part of the earth 
during the Carboniferous or Coal Age was composed chiefly of 
giant ferns and closely related forms. Seed plants as we know 
them today appeared much later. But the important point to 
keep in mind is that plants of the ast are the ancestors of those 
populating the earth now. There has been a gradual development 
of the plant kingdom extending over a period of many hundreds 
of thousands of years, and this process of development or unfolding 
is still going on today. 

Green plants are the great converters of solar energy. With- 
out them, animal life on the earth would be impossible. Green 
plants are the only organisms on the earth which have the power 
to convert light energy into food energy. Green plants alone can 
take materials from the soil and air, and with the aid of light, 
change these materials into food the food of both plants and 
animals. Animals and non-green plants must have their foods 
prepared for them; they are dependent organisms. Green plants, 
on the other hand, are independent in that they make their own 
foods. Just like any other manufacturing process, that of food- 
making by green plants requires energy; to build up foods from 
simple materials derived from the soil and air, energy is needed. 
This energy comes from light. And it is through the medium of 


green plants that light energy is transformed to food energy. 

When wood is burned, energy is liberated in the form of heat. 
In burning, the energy of the chemical compounds composing 
wood is transformed to heat energy. But in the building of these 
chemical compounds which compose food, light energy is necessary. 
Hence the energy set free in burning wood is in reality solar 
energy. When coal is burned there is liberated light energy which 
came to the earth hundreds of thousands of years ago, which en- 
ergy was utilized by the plants of that period, transformed into 
plant tissue, which later formed coal. 

When plants respire, there is a destruction of plant substance, 
accompanied by the liberation of energy. To build plant substance 
there is required, in the last analysis, solar energy. Hence, the 
energy freed in respiration is transformed solar energy. Is energy 
liberated when we, as humans, respire? Explain. 

Whatever way one may consider it, the fact remains that all 
life on this earth depends upon light energy and it is only green 
plants that are capable of transforming this light energy into a 
form which can be used by plants and animals alike as food. 

Plants are of great economic importance. They furnish food, 
clothing, and shelter. The great civilizations of the world have 
developed where natural conditions favored the cultivation of 
certain food plants, chiefly cereals. Consider the importance of 
such products of plant origin as wood, coal, cork, fiber, resins and 
turpentine, gums, plant dyes, fixed and volatile oils, and rubber. 
Many plants yield valued drugs, such as morphine, quinine, digi- 
talin, and atropine. Many species are of ornamental value, being 
employed to beautify our homes, gardens, and parks. A large 
number of plants are of economic importance because they are 
harmful, or interfere with man's operations. Consider here the 
plants which cause rusts, smuts, molds, mildews, and other plant 
diseases; also poisonous plants, hay-fever plants, and weeds. 

Botany, the science which deals with plants, is today an 
extremely important field of study. A knowledge of plants 
their structure, their behavior, their relation to the environment, 
their classification and naming, their improvement by breeding 
and selection, their relation to diseases of useful plants and of ani- 
mals is as essential to a proper understanding of agriculture in 


all its many branches, and to certain phases of medicine, as is 
mathematics to advancement in the field of engineering. For 
the individual seeking a life work, there are innumerable opportu- 
nities in the field of botany. In the educational institutions of 
the country universities, colleges, and high schools there are 
many technically trained botanists, specialists in some branch of 
plant science. These individuals are either teachers or research 
workers, or both. There are systematic botanists, plant mor- 
phologists, plant cytologists, plant geneticists, etc. In the United 
States Department of Agriculture, and in the agricultural experi- 
ment stations, of which there is one or more in each of the states, 
there are altogether several thousand workers, trained in some 
special field of botany. In addition to these, individuals equipped 
with a knowledge of some phase of plant science are found in the 
employ of botanical gardens, of museums, of national parks, of 
large companies which grow drug plants, sugar plants, nursery 
stock, seeds, rubber, tobacco, fruits, vegetables, fibers, and other 
industrial plants. 

In addition to these botany specialists, a certain knowledge of 
plants is usually required of those whose major interest may be in 
such fields as zoology, entomology, geology, pharmacology, animal 
husbandry, veterinary medicine, bacteriology, soil technology, 
irrigation practice, etc. (NOTE: If the student does not know 
what the above sciences treat of, he should attempt to find out. 
Consult dictionary or encyclopedia, or special books.) 

The student may gain some knowledge of the number of botan- 
ists in the United States, and the character of the positions they 
hold, from American Men of Science, a Biographical Directory, 
edited by J. McKeen and Jaques Cattell, and published by the 
Science Press, New York. The fifth edition has 1278 pages. 


Ernest H. Wilson, Plant Hunter, by EDWARD I. FARRINGTON, is a well- 
illustrated book of 187 pages, published by the Stratford Company, Boston. 
1931. This book describes the colorful adventures of Ernest Wilson in his 
search for plants in various parts of the world. 

The Geography of Plants, by M. E. Hardy, published by the Clarendon 
Press, Oxford, England. 1920. A brief description of the plant life char- 
acteristic of different parts of the world. 327 pages, 114 illustrations. 


An Outline of Plant Geography, by DOUGLAS HOUGHTON CAMPBELL, 
published by the Macmillan Company, New York. 1926. "For more than 
thirty years the writer has made excursions into many parts of the world, 
and the specimens, notes, sketches and photographs accumulated during 
these journeys have served as the basis of the present volume." 392 pages, 
153 illustrations. 

America's Greatest Garden, the Arnold Arboretum, by E. H. WILSON, 
published by the Stratford Company, Boston. 1925. This is "a note of 
invitation to a banquet of flowers and fruit provided by an assemblage of 
the World's best hardy trees and shrubs." 123 pages and 50 full-page illus- 
trations of great beauty and interest. 

Tree Ancestors, a Glimpse into the Past, by E. W. BERRY, published by 
Williams and Wilkins Company, Baltimore. 1923. The sketches of the 
book are "an attempt to interest the general public in the marvellous history 
of some of our trees." It discusses geological principles, methods of preserva- 
tion of fossil plants, geological time and methods of reckoning, the later 
geological history of North America, the present forests of North America, 
and the history of such trees as the sequoias, bald cypress, walnuts, beech, 
magnolia, -maple, ash, and many others. 270 pages and 48 illustrations. 

Plant Hunting on the Edge of the World, by F. KINGDOM WARD, published 
by Victor Gollancz, Ltd., Co vent Garden, London. 1930. This is a travel 
book with a strong botanical flavor. The author describes his journeys to 
"collect seeds of beautiful hardy flowering plants for English gardens, to collect 
dried specimens of interesting plants for study," and "to explore unknown 
mountain ranges and find out something about their past history, the distri- 
bution of their plants, and any other secrets they are willing to reveal." 
383 pages and 15 illustrations. 

Exploring for Plants, by DAVID FAIRCHILD, published by the Macmillan 
Company, New York. 1930. A most interesting book by one who for many 
years was in charge of the Office of Foreign Plant Introduction, of the U. S. 
Department of Agriculture. 591 pages and 179 illustrations. 

The Natural History of Plants, by ANTON KERNER and F. W. OLIVER. 
Published by Blackie and Son, London. 1895-96. In two volumes, Vol. I, 
777 pages; Vol. II, 983 pages, with about 2000 original woodcut illustrations 
A classical work replete with interesting facts about plants. 


The human body is made up of a number of organs, each with a 
work to do. There are organs of sight, of hearing, of digestion, of 
circulation, etc. The plant body, too, is composed of a number of 
organs, each having some definite work to do. For example, 
among seed plants those plants with which we are most familiar 
the roots are the absorbing and anchoring organs, the stems are 
the supporting and conducting organs, the leaves are the chief 
water-losing and food-making organs, and the flowers are the 
reproductive organs. But there are many plants much different 
from seed plants and in many respects simpler in their organiza- 
tion. For example, ferns have bodies with roots, stems, and a pecu- 
liar form of leaf which we call a " frond," (Fig. 177), but ferns do not 
have flowers and seeds. Mosses are prostrate plants, much simpler 
in their make-up than either seed plants or ferns; they have a 
very poorly developed conducting system, weak stems, extremely 
small leaves, no roots in the ordinary sense, and reproductive 
structures which have no resemblance to flowers. Still lower in 
the scale of plant life is that great group of plants which includes 
pond scums and seaweeds, bacteria and yeasts, rusts and smuts, 
molds and mildews, mushrooms and toadstools plants which 
have no roots, stems, or leaves, and very simple reproductive 
organs. There are even plants, bacteria and certain algae, the 
entire body of which consists of a single cell. A plant body of one 
cell is the simplest kind possible (Fig. 1, A, B, C, 3). 

Not only is there great variation in the structure of plants 
which compose the population of the world, but also there are 
very considerable differences in their chemical composition. For 
example, the sugar beet and sugar cane are richer in sugar than 
most other plants; oranges and lemons contain a relatively large 
amount of citric acid; in the seed of the castor-bean plant there is 
an oil known as castor oil; a latex, the basis of commercial rubber, 



exists in rubber plants; the bark of Cinchona yields a chemical 
known as quinine; the coffee berry produces an alkaloid caffein; 
tannins are chemical compounds derived from the bark of certain 
trees; and so on, there being literally thousands of chemicals 
manufactured in different plants. Name other chemical com- 
pounds derived from plants. 

Thus we see that, among the vast assemblage of plants which 
clothe the earth, there is great variation in their structure, that is, 
their organization, and in their chemical composition. 

Problem 1. What are the different forms of the plant body? 

When we consider the microscopic animal life of water and 
land, and the vast assemblage of insects, worms, Crustacea, rep- 
tiles, birds, and mammals, it would appear that almost every 
conceivable form of animal body is represented. Likewise in the 
plant kingdom is there tremendous variation in the forms of the 
plant body. The mention of a few plants will call to mind some 
of the different forms of body: seaweeds, yeast, bread mold, wheat 
smut, toadstools, liverworts, mosses, ferns, cycads, and the great 
variety of herbs, shrubs, and trees. 

The simplest plant body is a single cell. Among such simple 
one-celled plants are the bacteria. A plant body in which cells 
are joined end to end to form a thread represents the next stage 
of advance in complexity over the one-celled forms. There are 
many such thread-like plants among the pond scums, seaweeds, 
and certain fungi. Then there is a grouping of cells to make up 
such simple plant bodies as toadstools and mushrooms; these are 
many-celled plants, but devoid of roots, stems, leaves, and flowers. 
Still more complex plant bodies are those of flowering plants which 
have many different tissues and organs with which to carry on 
the work of the body. 

Thallus plants. There is a large group of plants known as the 
Thallophyta (thallus plants), to which belong the algae and the 
fungi. They are primitive members of the plant kingdom. 
The plant body is either a single cell or a simple grouping of cells 
to form a body that has no leaves, stems, or roots. Moreover, 
they do not have flowers, fruits, or seeds. 



l~- Spore 

FIG. 1. Different kinds of algae. A, 
Gloeocapsa, a blue-green alga in which 
the individuals are surrounded by a 
gelatinous coating; B, Synechococcus, 
a blue-green alga; C, Protococcus, a 
simple green alga; D, Oscillat&ria t a 
thread-like blue-green alga ; E, Nostoc, 
another thread-like blue-green alga; 
with germinating spore at left. (From 
Holman and Robbins, in a Textbook 
of General Botany.) 


Algae. Algae are plants of simple structure which grow in the 
water or in very moist situations. We are familiar with those that 


form a greenish slime on the sides and bottom of watering troughs 
and drinking fountains; and those that appear as a green coating 
on the north sides of trees in moist forests; and those that form a 
green, frothy, repulsive-looking scum on the water of ditches, 
ponds, reservoirs, and stagnant pools. Algae are found in both 
fresh and salt water. Many of the larger kinds in the ocean are 
known as " seaweeds." 

Four different groups of algae. As to color, there are four 
groups of algae: blue-green, green, brown, and red. 

The blue-green algae and green algae are chiefly of fresh waters, 
whereas the brown algae and the red algae are principally of salt 
waters. The brown algae, or brown seaweeds, are common along 
the shores of all oceans. They are attached by specially modified 
structures, holdfasts, to pilings, rocks, etc. The best-known brown 
algae are the giant kelps, some of which may reach a length of 
800 to 900 feet; the rockweeds, which are found on the rocks 
between high-tide and low-tide marks; and Sargassum, which 
becomes detached from its growing places along shores, and is 
often carried far into the ocean. The " Sargasso Sea " in the 
North Atlantic Ocean is a floating mass of the brown alga, Sargas- 
sum, carried there by ocean currents from distant shores. It is 
recorded that Columbus saw the Sargasso Sea on his memorable 
voyage to the New World. William Beebe and Ruth Rose in 
The Arcturus Adventure, a Putnam publication, describe in a most 
interesting manner the " Sargasso weeds and waves." 

The red algae, or red seaweeds, are quite beautiful, delicate 
plants and are usually much smaller than the brown algae. The 
plants are often very highly branched, the divisions being fine and 
thread-like. The red algae are found in deeper waters than the 
brown algae. 

The simplest algae, such as Gloeocapsa, and Protococcus, are 
one-celled plants. The whole plant consists of but one spherical 
cell. What simpler plant could there be? But these microscopic 
one-celled plants carry on all the life processes, such as absorption 
of water and mineral nutrients which are taken in at all points on 
the plant body, respiration, food manufacture, digestion, assimila- 
tion, and reproduction. 

Many of the algae are filamentous or thread-like forms. That 


is, their bodies consist of a single chain of cells. Common examples 
of filamentous algae are : Nostoc and Oscillatoria of the blue-green 
algae, and Spirogyra and Ulothrix of the green algae. 

Nostoc plants are often aggregated to form bluish-green balls, 
which may be found on damp earth or in water. The threads or 
filaments (the plants) are embedded in, and held together by, 
a gelatinous material secreted by the different cells that make 
up the colony. Chlorophyll is present, and with it is a blue-green 
pigment which gives the blue-green color to the whole cell. 

Spirogyra or common pond scum is one of the most widely dis- 
tributed of the green algae. Each cell as shown in Fig. 2 is a 
short cylinder, with well-defined walls of cellulose. Each cell has 
one or more conspicuous spiral chlorophyll bands. The spiral band 

FIG. 2. Drawing showing the structure of a cell of Spirogyra and its rela- 
tion to other cells of a filament. The cell wall is lined by a thin layer of 
cytoplasm which holds a spiral chloroplast. This bag of cytoplasm is filled 
with cell sap within which is suspended a small mass of cytoplasm containing 
the nucleus of the cell. 

is a specialized mass of living material. In addition, there will be 
found near the center of the cell a nucleus, and from it strands of 
protoplasm radiating to and connecting with the protoplasm that 
lines the wall. 

A number of algae, such as Cladophora, are branching, filament- 
ous forms. Some, like f/foa, consist of a single plate of cells. The 
brown and red algae, the " seaweeds," however, possess the most 
complex structure of all the algae. Their bodies may be large. 
In one of the rockweeds (Ascophyllum) , for example, the plant pos- 
sesses special holdfasts; it is highly branched, and the branches are 
of two different kinds; there are two quite distinct systems of 
tissues; and its reproductive organs are more complex than those 
in blue-green and green algae. 


Thus we see that algae, as a group, vary considerably in struc- 
ture. The blue-green algae include the simplest forms, many of 
them being simple one-celled plants. Some of the blue-green 
algae are filamentous. The green algae include one-celled, fila- 
mentous, and plate forms. The brown and red algae (" sea- 
weeds ") are often very large plants. 

Exercise 1. Different kinds of algae. Make a microscopic study of the 
different forms of algae which may be collected from ponds, streams, foun- 
tains, and moist surfaces of trees and rocks. Observe principally the varia- 
tions in the form of the plant body. Also, if possible, examine various kinds 
of brown and red seaweeds. 

Fungi. The fungi are forms of plant life which have no chloro- 
phyll and hence must secure their food ready-made from living 
organisms or from substances which were once a part of the bodies 
of living organisms. The foods of fungi, as of all plants and 
animals, are chiefly carbohydrates, fats, and proteins. As we have 
learned, green plants have the power of manufacturing these foods 
from carbon dioxide, water, and various mineral salts, that is, from 
inorganic materials. But the fungi, lacking chlorophyll, do not 
have this ability. They are dependent either directly or indirectly 
upon green plants. 

Those fungi which gain their foods from living plants or animals 
are called parasites; those which take their foods from the dead 
remains or products of living plants or animals are called sapro- 
phytes. For example, the rusts and smuts which gain nourish- 
ment from live tissues are parasites; and the molds of bread and 
fruits, and the various fungi which grow on decayed logs are 
saprophytes. The term host refers to the plant or animal from 
which the parasite derives nourishment. 

Different kinds of fungi. There are many thousand different 
kinds of fungi, and they affect man's welfare in many ways. It is 
difficult to overemphasize their importance. A number of bacteria 
and other fungi bring about decomposition of organic material, 
and are necessary to maintain soil fertility; thousands of them 
cause diseases of plants and animals, including man; they are 
essential in the making of cheese and of bread, in the retting of 
flax, and in many other commercial processes. 

We shall discuss briefly here a few of the most important 



groups of fungi, namely, bacteria, molds, mildews, yeast, smuts, 
rusts, mushrooms, and toadstools. 

The bacteria (Fig. 3). Bacteria teem in countless millions in 
the air, in the soil, and in the water; they are present upon the 
surface of the human body, and that of other animals, and in the 
intestinal tract; they abound in sewage, and in all decaying mate- 
rial ; they occur in the surface of all objects about us. Bacteria are 
the smallest known organisms. The average size is about 25 
inch in diameter. 

There are three principal types as 
to shape (Fig. 3): the spherical (coccus), 
the rod (bacillus), and the spiral (spiril- 

Exercise 2. Bacteriological laboratory. If 

possible the student, either alone, or with the 
class, should visit a bacteriological laboratory. 
In all cities of any size there is such a laboratory 
connected with the city health department. 
Observe here the equipment, methods, bacterial 
cultures, etc. Take the opportunity of observ- 
ing bacteria under an oil-immersion lens. 

-Three forms of 
bacteria. A, bacillus 
forms; B, coccus forms; 
C, spirillum forms. 

The molds. These usually form a 
cobwebby growth, and they occur on a 

great variety of organic materials, such as stale bread, fruit and 
vegetables, jellies, old leather, cheese, and moist paper. Fruit 
and vegetables in the market, in storage, or during shipment 
may be greatly damaged by these saprophytic fungi, especially if 
the air is warm and moist. To prevent their molding, such prod- 
ucts are shipped and stored at low temperatures, in refrigerator 

There are many different kinds of molds varying as to the 
color of the spores they produce: black, blue, brown, green, and 

The mildews. There are two different groups of mildews, the 
downy mildews, and the powdery mildews. Both groups are 
parasitic. The mildews of the first group form a downy white 
growth upon the surface of leaves, and sometimes on that of 
stems and fruits. The parasite is not confined to the surface, how- 



ever, but its threads enter the tissues and absorb food from them. 
The spores are borne in abundance on the surface of the host, and 
under suitable conditions will germinate immediately. Well-known 
destructive downy mildews are those causing the late blight of 
potato, and the mildew of grape, onion, lettuce, lima beans, cucum- 
ber, pumpkin, and watermelon. 

The rusts. The rust fungi constitute a very large group of 
plants, all of which are parasites, and many of which are of great 

/Conidiophores \ 



FIG. 4. Blue and green molds. A and B, common blue mold; C, germinating 

spore of blue mold; D, green mold. An hypha is a fungous thread; conid- 

iophore is a spore-bearing branch; conidiospore is a special type of spore. 

(From Holman and Robbins, in A Textbook of General Botany.) 

economic importance on account of their destruction of crop 
plants. Among the most important rusts are those of the cereals, 
asparagus, apple, raspberry, and pine. The black stem rust of 
wheat, oats, barley, rye, and other grasses has caused damage to 
the cereal crops amounting to millions of dollars, and at times has 
become epidemic. In the United States in 1916, the black stem 


rust (Fig. 43) caused a loss of wheat amounting to 200,000,000 
bushels. The white pine blister rust has threatened the destruc- 
tion of the white pine forests, the timber of which is valued at 

The smuts. All the smut fungi (Fig. 5) are parasites, occur- 
ring chiefly on members of the grass family. The annual losses of 
cereal crops due to the smuts frequently amount to 150,000,000 
bushels. The smuts may be recognized by the black masses of 

FIG. 5. Ear of corn showing a mild attack of corn smut. Corn smut is a 
parasite, deriving its food from the tissues of the living corn plant. 

spores. In cereal smuts these usually develop in the head and 
mature at about the same time the head matures. 

Mushrooms and toadstoools. These are fleshy fungi which 
are found growing in fields, pastures, and woodlands, and also 
upon decaying logs and tree trunks. There is a great variety of 
" mushrooms " and " toadstools " (Fig. 6). Probably the best 
known are those which bear gills and are known as the gill fungi 
or " agarics." Others are the pore fungi, the tooth fungi, the 
carrion fungi, and puffballs. 



Let us describe very briefly the common " meadow mushroom/' 
the familiar edible one. The " mushroom " as we see it consists 
of a stalk and an umbrella-shaped cap. On the under side of the 
cap are thin gills. The spores are borne in enormous numbers on 
the surfaces of these gills. Each spore is a microscopic spherical 
body, light in weight, and capable of being carried long distances by 
air currents. The cap with its stalk, constituting the mushroom, 
forms in reality only a small part of the plant body. The mush- 
room is a fruiting body and arises from a great mass of fungous 

FIG. 6. Longitudinal section of a mush- 
room (Tricholoma). The umbrella-shaped 
cap, or pileus, from which the spore-bearing 
gills hang, is supported by the stalk, or stipe. 

FIG. 7. Puffballs, showing 

some of the underground 

hypha threads. 

threads which are distributed in the soil, and which gain their food 
from decayed organic matter. 

The so-called mushroom " spawn " sold by seedsmen usually 
consists of dried manure containing the fungous threads, all being 
pressed together in brick form. When a mushroom bed is made, 
the spawn is broken up, mixed with earth, and used to start the 
beds. The mushroom originates from the fungous threads in the 

In the tooth fungi there are teeth or spines on the under side 
of the cap, and these bear the spores. In the pore fungi the spores 


are borne in open tubes or pits on the under side of the cap. A 
number of pore fungi cause the rotting of wood. The puffballs 
(Fig. 7) rupture when mature, setting free black clouds of spores. 
A number of the fleshy fungi are poisonous. Although there 
is no botanical difference between " mushrooms " and " toad- 
stools," the former name is commonly applied to those believed 
to be edible, and the latter to those thought to be poisonous. 

Exercise 3. The plant body of different fungi. Observe the plant bodies 
of a variety of fungi such as molds of bread and fruit, yeast, mildews, smuts, 
rusts, toadstools, and mushrooms. In what respects are they alike? In 
what respects do they differ? 

Mosses and Liverworts. The name " moss " is applied popu- 
larly to a number of different kinds of plants. Some of the sea- 
weeds are called " sea mosses," but the true mosses never occur 
in saline waters. The "Spanish moss" (Tillandsia) which hangs 
from the trees in our southern swamps is not a true moss but a 
flowering plant. "Reindeer moss" is a lichen, as is also the 
"moss" that hangs from the limbs of conifers in the northern 
states and in the high mountains of the West. 

The true mosses are low plants seldom more than a few inches 
in height, with an erect stem, upon which very small leaves are 
densely crowded. The leaves are usually but one cell in thickness, 
except along the midrib and sometimes around the margin. There 
are no true roots in mosses. They possess structures known as 
rhizoids, which, although they have not the structure of roots, serve 
the same purposes of absorption and anchorage. Identify differ- 
ent moss structures shown in Fig. 104. 

A distinctive feature of mosses is the " fruiting " or spore- 
producing body. This is a spore-case or capsule (Fig. 104) at the 
tip of a stalk. Numerous spores are borne within this capsule. 

Mosses are found chiefly in moist woods and in swamps, but 
some species occur on the bark of trees and in dry rock crevices. 
In regions with a prolonged moist season, they may be seen grow- 
ing on fences, and on the shingle roof of old buildings. They 
are conspicuous on account of the " carpet " or mass of vegeta- 
tion they form. 

The mosses are divided into three distinct groups: (1) the peat 
mosses, (2) the bkck mosses, and (3) the true mosses. They 



differ somewhat in their appearance, structure, habits, and life 

The liverworts (Fig. 107) are low-growing plants, chiefly found 
in moist places. The plant body is thin, green, and flat against 
the ground, being attached to the soil by slender root-like 
structures, known as rhizoids. Marchantia is a well-known liver- 
wort, the body of which is more or less lobed. There are 
certain leafy liverworts, the body of which is composed of a 
slender prostrate axis or "stem" bearing three crowded rows of 

small leaf -like structures. 
The body is attached to the 
soil by rhizoids. 

Exercise 4. The plant bodies 
of mosses and liverworts. Ob- 
serve in the field or greenhouse 
the plant bodies of different 
mosses and liverworts. Contrast 
them with those of the thallus 
plants, enumerating differences. 

The ferns and their allies. 

The ferns and their close re- 
latives, the club mosses and 
scouring rushes (horsetails) 
(Fig. 8), constitute a large 
group of plants. Like the 
algae, fungi, and mosses, 
they reproduce by means 
of spores, but unlike these 
groups, they possess woody 
stems and roots, and a con- 
ducting tissue, similar to 
that in flowering plants. 
The ferns and their allies 

FIG. 8. Horsetail or scouring rush. A, 
early spring stems arising from rootstock; 
note the scale-like leaves at the joints and 
the spore-bearing cones at the tip; B, 
branching form which appears later in the 
season than the preceding. (From Glover 
and Robbins, in Colo. Agr. Exp. Station 

do not produce flowers. 
In the common cultivated ferns, the stem system is wholly 
under ground. It persists from year to year, growing in length at 
the tip, branching somewhat, and sending into the air each season 
a number of leaves, the so-called fronds. After a time there appear 




FIG. 9. The stag-horn fern. 

on the under side of the frond brownish groups of spores which are 

often mistaken for some disease or insect. They are, however, the 

reproductive bodies. The 

fern-lover should read the 

article " Ferns as a Hobby " 

by William R. Maxon, in 

the National Geographic 

Magazine, Vol. 47, pages 

541-586, 1925. 
The horsetails or scouring 

rushes have harsh, jointed 

stems which arise from a 

rootstock. The leaves are 

mere scales. The spores are 

borne in a cone at the tip of certain branches. On account of 

their harsh texture, the plants have been used for cleaning and 

polishing utensils. They are re- 
puted to be poisonous to live- 
stock, chiefly horses. Sometimes 
they behave as weeds. 

The club mosses are usually 
creeping or trailing plants, some- 
times known as " ground pine " or 
"running cypress." The spores are 
borne in leafy cones at the tips of 
branches. The spores are sold in 
drug stores under the name " ly co- 
podium powder," and are used as a 
drying powder and to some extent 
in the manufacture of fireworks. 

FIG. 10. Ferns and club mosses in 

the Garfield Park Conservatory, 


Exercise 5. Plant bodies of ferns 
and their allies. Observe in the field 
or greenhouse the plant bodies of 
different kinds of ferns, club mosses, 

and horsetails. Enumerate the differences between the plant bodies of ferns 

and their allies, and those of mosses and liverworts. 

Seed plants. The seed plants possess the most complex body 
of all plants. There are many different organs and tissues. There 



are roots, stems, leaves, flowers, fruit, and seed, except in one large 
group, the Gymnosperms, which have no flowers in the ordinary 

FIG. 11. Wheat plant showing the general habit of growth of grasses. (From 
Robbins, in Botany of Crop Plants.) 



sense, and there is tremendous variation in the form of these 
organs. Moreover, there are many different kinds of tissues which 
compose these various organs. 

FIG. 12. The Deodar Cedar, a seed plant. In this and other conifers there 
is a "leader" one main stem which throughout the life of the plant holds 

this leadership. 

Fia. 13. The Live Oak, a seed plant. Compare the branching habit of 

this plant with that of the Deodar in Fig. 12. The form of the plant body is 

largely determined by its branching habit. 

Exercise 6. The seed-plant body. The student should take a field trip 
and note the different forms of seed-plant body. The various species of trees 
and shrubs have characteristic shapes; these may be shown well by quickly 



Cell Wall 



sketching their outline. Observe not only erect forms of plants, but also 
climbing and creeping forms. Also note the form of plant bodies growing 
under different environmental conditions. 

Problem 2. What is the structure of the plant cell? 

If we study with the microscope the structure of plant tissues, 
we find them to be made up of many small bags or sacs with walls 

which are usually thin and trans- 
parent. Each of these microscopic 
sacs or compartments is called a cell 
(Fig. 14). The term "cell" was 
first used by Robert Hooke, an 
Englishman who lived from 1636 to 
1703. With his improved micro- 
scope he examined all sorts of things, 
among them ordinary bottle cork. 
He observed this plant tissue to be 
made up of numerous compartments 
resembling the cells of honeycomb. 
So Hooke named the compartments 

Just as a brick house is made up 
of separate units, the bricks, so is the 
plant body made of separate units, 
the cells. It is in the cells that all 
the complex physical and chemical 
changes of the living body go on. 
Careful observation of plant cells 
under the microscope reveals that 
within each of the cells there is a 
quantity of a jelly-like substance. 
This is the living material and is called 

protoplasm. What is the literal meaning of the term protoplasm? 
The protoplasm of the cell is not of the same structure through- 
out. A denser mass of living material, the nucleus, is usually 
prominent in the cell. The nucleus is a very important part of the 
cell, taking an active part when the cell divides. Most important of 
all, the nucleus carries those determiners of characteristics which are 


'' "' '''AT- <' - A" ^ n "j. - 

fr~*&*.&f* s &ii&i ^ *: 

FIG. 14. Two young cells 
from the growing point of a 
root. (From Holman and 
Robbins, in A Textbook of 
General Botany.) 


passed on from cell to cell, that is, from parent to offspring. In 
addition to the nucleus, there may be other specialized masses of 
protoplasm in the cell, known as plastids; chief of these are the 
green plastids (chloroplastids) which are the centers of the process of 
carbohydrate manufacture. All the protoplasm of the cell outside of 
the nucleus is called cytoplasm. Protoplasm is a mixture of many 
different chemical compounds, some of which are exceedingly 

In addition to the living substance the cell contains much 
material that is not alive. For example, every cell contains water 
in which are dissolved various substances that have come from the 
soil, and certain foods, such as sugar, which have been manufac- 
tured in the leaves. Sap is the name we apply to the water of 
the cells plus the various substances which are dissolved in it. 
In other words, cell sap is a solution, in which water is the solvent. 
The cell may also contain substances such as starch and protein 
which are not soluble in the water of the cell. The wall about the 
cell is not alive. It is made up of a material called cellulose, a sub- 
stance closely related in its chemical composition to starch and 
sugar. Cellulose, like starch and sugar, is made up of but three 
elementary substances, carbon, oxygen, and hydrogen. Cellulose 
is the most abundant plant substance in the world. It is of interest 
to note here, in passing, that cotton, linen, hemp, and wood consist 
of cellulose, and that it is used as a raw material in the manufacture 
of such substances as artificial silk, paper, celluloid, cellophane, and 
guncotton. The inquiring student will want to find out how these 
materials, as well as many others, are manufactured from cellulose. 

Exercise 7. The cells of soft tissue. Examine the soft tissues of broken 
leaves, vegetables, fruits, stems, etc., with a binocular dissecting microscope. 
It will be possible even with a magnification of 20 diameters to see that the 
tissues are composed of many small compartments, varying considerably in 
shape. These compartments are the cells. They are the units of structure. 

Exercise 8. Cells in the leaf of Elodea, a water plant. The leaves of the 
water plant Elodea are exceedingly thin, mostly one layer of cells thick. 
Mount a single leaf flat in a drop of water on a slide, and cover with cover- 
slip. Examine with compound microscope. Compare what you see with 
Fig. 15. The cells are filled with green plastids (chloroplastids), which may 
wholly or partly obscure the more transparent nucleus. The walls are of 
cellulose and thin. Observe the different shapes of cells in different parts of 
the leaf. In fresh, young leaves, the student will observe a movement or 



streaming of the protoplasm, the chloroplastids being carried along with the 
stream, as chips of wood in flowing water. The chloroplastids themselves are 
living bodies. It is in them that glucose, a sugar, is manufactured, with the 
aid of light. In all living cells there is a movement of the protoplasm, but in 
only a few plants is it rapid enough to be readily observed. What do you 
believe to be the advantage of this protoplasmic movement? 

Exercise 9. Storage cells of the potato. Cut very thin sections of the 
inside white tissue of a potato tuber. Mount in water and examine with 
compound microscope. Observe the large, thin-walled cells, rilled with starch 

FIG. 15. View of a portion 
of an Elodea leaf as seen 
under the high power of the 
microscope. The cell struc- 
ture is similar to that of 
onion skin (Fig. 16). How- 
ever, as these cells are from 
a green leaf, they contain 
chloroplasts which float in 
the cytoplasm. While the 
nucleus can be shown to be 
present by staining, yet it 
is not easily seen in the 
fresh material. 

FIG. 16. View of onion skin 
as it appears under the micro- 
scope. Note the well-defined 
cell wall. Within the cell wall 
there is a thin layer of cyto- 
plasm which, together with the 
small disk-shaped nucleus, is 
made up of protoplasm, the 
living matter of the cell. Cell 
sap fills the cavity within the 

grains. Starch is a non-living substance a storage product. It is of interest 
to know that potatoes which are mealy when cooked are those in which the 
cells are well filled with starch; whereas in watery potatoes starch grains do 
not fill the cells. Americans prefer mealy potatoes, but Frenchmen as a rule 
prefer the more watery sorts. Name other plants that store large amounts of 
starch in, some part of the plant. 


Cell Wall 
Cell Cavity 
Intercellular Space 

FIG. 17. Different kinds of cells and tissues. A, fibers as seen in cross- 
section; B, fibers as seen in lengthwise section; C, a single fiber; D, stone 
cells from the shell of English walnut; E, cork cells; F, a food-manufacturing 
cell from a leaf; G, thick-walled pitted cells from endosperm of asparagus 
seed; H, starch-storing cell; I, different types of vessels; J, a simple pitted 
tracheid from pine wood ; K, tissue made of thin-walled cells which fit loosely 
together. (Except A, B, C, and G, from Holman and Robbins, in A Textbook 

of General Botany.) 


Exercise 10. Different kinds of cells. The inquiring student will be 
interested in examining the tissues of different organs of ordinary flowering 
plants, and also those of lower plants, such as algae, mosses, liverworts, ferns, 
etc. See Fig. 17. He will see cells differing in size, in shape, in the thickness 
and markings of the walls, and in the nature of their contents. 

Problem 3. What is the nature of protoplasm the living 


In 1590, two Dutch brothers, spectacle-makers, invented the 
compound microscope, an instrument which was destined to 
become the most important tool of biological science a tool 
which has made possible much of the progress in our knowledge 
of plants and animals, of medicine, of agriculture, of heredity. 

As stated above, Robert Hooke greatly improved the micro- 
scope, and examined with great curiosity all sorts of things, among 
them bottle cork. Hooke saw in the cork tissue only the walls of 
dead cells. He had no clear idea of the cell contents. It was not 
until 1831 that another Englishman, Robert Brown, first recog- 
nized the importance of the nucleus in the cell, and not until 1861 
that Max Schultze, a German, established the close similarity of 
the living substance of plants and of animals, and formulated what 
is known as the protoplasmic doctrine which says that the essen- 
tial part of a cell, the part which is responsible for its life, is the 
protoplasm. The unit of structure and activity is really a highly 
organized protoplasmic mass; the wall is merely a non-living, 
enclosing shell. Protoplasm has well been called the physical basis 
of life. 

Physical properties of protoplasm. Protoplasm is a semi- 
transparent, slime-like substance, much the consistency of the 
white of an egg. However, it does not have the same appearance 
throughout all parts of the cell; the nucleus is much denser and 
is darker than the cytoplasm as seen in stained cells; also there 
are small, dark granules embedded in the protoplasm, some 
of which are living; and, as we have learned, there are larger, 
living bodies, the plastids, floating in the mass of protoplasm. 
Sometimes these plastids contain a green pigment (chlorophyll), 
sometimes orange and red pigments (carotin and xanthophyll). 
Is it true that the green color of the vegetation of the world is due 


to the pigment chlorophyll? When a cell dies the protoplasm loses 
its liquid consistency and coagulates, that is, sets into a more or 
less firm mass, like the white of an egg when it is boiled. 

Exercise 11. The living protoplasm in the cells of squash and Elodea. 
Mount in water the hairs found on the stems, near the tip, of the squash plant. 
Under the high power of the microscope one will see, in certain cells of the 
hairs, a grayish, semi-transparent substance, the protoplasm. The nucleus is 
darker gray, and leading from it are strands or threads of cytoplasm. Cyto- 
plasm also lines the wall of the cell. Mount fresh leaves of Elodea, as you did 
in Exercise 8, Problem 2. The cells are filled with green plastids (chloro- 
plastids), which are embedded in _a gray cytoplasm. Each plastid is a jelly- 
like mass of protoplasm, and dissolved within it is a pigment known as 
chlorophyll. See Fig. 15. 

Chemical properties of protoplasm. Protoplasm appears to 
be a mixture of a number of different chemical compounds. It is 
not a single compound like sugar. We can write the chemical for- 
mula of cane sugar as follows: Ci2H220n; but we cannot write a 
chemical formula for protoplasm. The following chemical ele- 
ments are always found when protoplasm is analyzed: carbon, 
hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 
magnesium, and calcium. There is no chemical element found in 
protoplasm that does not occur in the soil and air. The compounds 
occurring in the mixture of compounds composing protoplasm 
include principally different proteins, fatty substances known as 
lipoids, carbohydrates, and salts. Usually 80 per cent, by weight, 
or more of protoplasm is water. Proteins rank second, by weight. 
The proportion of the different compounds making up the proto- 
plasm of any cell is constantly changing. Furthermore, there 
appear to be significant differences between the chemical composi- 
tion of the protoplasm of different kinds of plants, and of different 
cells of the same plant. Generally speaking, protoplasm, the liv- 
ing material, is a very complex mixture of chemical compounds, 
including among them proteins, known to be in themselves the 
most complex chemical compounds thus far analyzed. (Refer to 
Chemical Phenomena in Life, by Frederick Czapek, 152 pages, 
published by Harper and Brothers, New York.) 

Physiological properties of protoplasm. Protoplasm, the living 
material, possesses certain properties and powers that are peculiar 
to it and that distinguish it from non-living material. For exam- 


pie, a cell grows and develops new cells; that is, it has the power 
of growth and reproduction. In order to grow and reproduce, 
the cell must secure material from outside of itself and make that 
material a part of itself; that is, it must synthesize carbohydrates, 
fats, proteins, and other substances, and then change these mate- 
rials into living stuff. This change of non-living stuff to living is 
known as assimilation, a change the nature of which is but poorly 
understood. We have come to believe that all life comes from 
pre-existing life; that is, new cells and new organisms are de- 
rived only from existing ones. In the processes of growth and 
reproduction, the protoplasm throws off waste products, that is, 
it has the power of excretion. For the building processes occurring 
in protoplasm, there must be a supply of energy. This the cell 
secures through respiration, another one of its peculiar processes, 
in which complex substances are broken down and energy liber- 
ated. Still another physiological property of protoplasm is that of 
irritability. By this we mean its ability to " sense " stimuli and 
respond to them. Protoplasm " perceives " light, gravity, water, 
and other external factors, and responds to them. Have you 
observed that plants in a window grow toward the light? See 
Fig. 128. That roots grow downward in response to gravity? 
That roots grow into moist soil rather than into dry soil? Proto- 
plasm is irritable. 

Suggested activities, (a) Is there spontaneous origin of life? The student 
should read a short account and prepare a report on how man was led, through 
his discoveries, to overthrow the notion that life might originate spontaneously. 
See one of the following: (1) Locy's Biology and Its Makers, (2) DeKruif's 
Microbe Hunters. (6) Devise an experiment to show that plants are respon- 
sive to light, (c) Devise another experiment demonstrating that roots 
respond to gravity. 

Problem 4. How are cells grouped to form tissues and organs? 

The cells of the plant body vary a great deal in size, in shape, in 
age, in the kind of material they contain, and in the nature of the 
work they have to do. It would take about one thousand average- 
sized sugar storage cells, from the root of a sugar beet, placed side 
by side to make an inch. Many cells of the plant are box-shaped 
with either square or rounded corners, others are spherical, and 


still others are much longer than wide, and with pointed ends. In 
the growing points of the plant, as in buds and at root tips, the 
cells are much younger than those found farther removed from 
these tips. 

There are cells in the plant body adapted to carry on the dif- 
ferent kinds of work it has to do. Some are fitted to absorb water 
and mineral salts from the soil; others carry water, mineral salts, 
and foods from one part of the plant to another; others are spe- 
cially fitted to manufacture food; many cells act as storage 
reservoirs of food; and still others are chiefly concerned with 

The cells of the plant body having special kinds of work to do 
are usually grouped together, forming tissues. These tissues are 
given names describing the functions or kinds of work they per- 
form. For example, there are absorptive tissue, conductive tissue, 
strengthening tissue, food-making tissue, storage tissue, and repro- 
ductive tissue. An organ may be composed of several kinds of 
tissue. For example, a leaf possesses conductive and strengthen- 
ing tissue in its veins, food-making tissue in the softer parts, and 
a protective tissue (epidermis) which covers the entire surface. 

Thus we see that the living plant body, like the human body, 
is a complex structure, composed of innumerable units, the cells, 
grouped together to form tissues, each with a special work to do, 
and the tissues are in turn grouped to form the organs of the plant. 
We may picture the healthy, living plant as a marvelously con- 
structed body, in which there is a splendid division of labor, with 
all cells, tissues, and organs working in harmony. We come to 
realize that living things resemble each other not only in gross 
structure and function, but also in microscopic structure. 

Exercise 12. Different kinds of tissues. The student should examine 
thin sections of different kinds of plant materials and observe how the cells 
are grouped to form tissues and organs. How do cells differ in shape? Com- 
pare different tissues as to hardness, compactness, and strength. 

Suggested activity. Make plasticene models of different kinds of cells. 

Problem 6. What is the relation of structure and function? 

The living plant expends energy and does work. If it were 
possible to take moving pictures of the plant at work, and to run 


the film at high speed, we would see the roots twisting and turning, 
making their way about, and between the soil particles we would 
observe the young sprout of the germinating seed straining to lift 
the load of soil from its path ; we would see parts of opening buds 
moving vigorously; in fact, there would be active movement 
throughout the plant body; and if we examined the interior of the 
cells, we would see the living material, the protoplasm, moving in 
streams from one part of the cell to the other. Plants, like animals, 
actually do work as long as they are alive, and this work requires a 
supply of energy. 

The various activities or kinds of work performed by plants are 
spoken of as their functions. For example, absorption of water 
and salts from the soil, movement of materials in the plant tissues, 
manufacture of foods in green tissues, digestion of foods, loss of 
water, respiration, and reproduction are all functions of the plant 
body. Clearly, these functions are associated with certain struc- 
tures. For instance, absorption of water and salts is associated 
with roots, loss of water chiefly with leaves, manufacture of food 
chiefly with leaves, etc. Furthermore, the organs are so con- 
structed as to carry on well the particular functions associated 
with them. 

Let us consider the function, photosynthesis, the manufacture 
of sugar in the leaf. Is the leaf structure as shown in Fig. 29 
well fitted to carry on this process? In the process of photo- 
synthesis carbon dioxide gas from the atmosphere and water from 
the soil are brought together in green leaf cells, and there, with the 
aid of light, built into a sugar, glucose. The manufacturing cells 
of the leaf require water, carbon dioxide gas, and light. It is 
obvious that a thin, flat, expanded structure, like a leaf, exposes 
a large surface for the absorption of both light and carbon dioxide. 
As concerns light absorption, the epidermis of the leaf is thin and 
transparent, and thus allows light to penetrate the underlying 
tissues; and the internal cells possess a pigment, chlorophyll, which 
is effective in absorbing light energy. As concerns carbon dioxide 
absorption, the epidermis of the leaf has numerous small openings 
or pores (stomata) which permit the movement inward of gases; 
furthermore, inside of the leaf the cells fit together very loosely, 
leaving large air spaces which facilitate the movement of carbon 


dioxide in the leaf, allowing it to come in contact with the surfaces 
of almost all cells, by which it is absorbed. There is a network of 
fine veins in the leaf, which bring water to the food-making cells. 
No food-making cell is far removed from a water-conducting vein. 
Thus, it would appear that the leaf is a structure well adapted to 
carry on the function of photosynthesis or manufacture of sugar. 
It would be possible to show how many other organs and tissues 
of the plant body are adapted for the specific work they perform. 
The relationship between structure and function is a close one. 
Thought on the part of the student will bring to mind many of 
these relationships in the plant. 

Exercise 13. Relation of structure and function. Discuss the relation- 
ship between the following structures and functions: (a) waxy coating of leaf 
surface and loss of water from the leaf; (6) hairs on leaf surface and loss of 
water from leaf; (c) thin cellulose wall of root hair and absorption; (d) roots 
and absorption of materials from the soil; (e) leaf and food-manufacture. 

Problem 6. What are the chemical substances found in plants? 

In his study of all sorts of food and forage and medicinal plants, 
the scientist has made a great many chemical analyses. These 
analyses have told him much of the economic value of plants, of 
the relative importance of different parts of the same plant, and 
also have thrown light upon the movement of materials in the 
plant, and the influence of various environmental factors, such as 
fertilizers, upon the composition of the plant. For example, he 
knows from such chemical studies that some varieties of wheat 
are richer in protein than others, that the most starchy and valu- 
able part of the potato tuber is the cortex or that portion just be- 
neath the skin, that the food made use of by the opening fruit 
buds of orchard plants is stored in woody tissue not far removed 
from these buds, that an excess of nitrates as compared with carbo- 
hydrates in a tomato plant suppresses fruit development, that a 
deficiency of iron in the plant causes it to be pale and sickly, that 
sugar beets growing in a soil with a too liberal supply of nitrogen- 
carrying fertilizers are usually low in sugar content, and many 
other valuable and interesting facts about plants. There are sev- 
eral hundred plant chemists in the laboratories of this country. 


Analyses have detected in plants no chemical elements except 
those found in the soil or air. That is, there are no chemical ele- 
ments peculiar to plants. The principal elements which compose 
the living and non-living parts of plants are carbon, hydrogen, 
oxygen, nitrogen, phosphorus, sulphur, potassium, calcium, mag- 
nesium, iron, and silicon. These are common elements found in the 
minerals of the soil. Of course, many more chemical elements 
than those listed above occur in plants. The different materials 
of the soil are not absorbed in equal amounts by different kinds of 
plants. Plants have some " selective power/' We well know 
that if two different kinds of plants, peas and tomatoes, for exam- 
ple, are growing in the same soil, their mineral composition is not 
the same. Doesn't this show that different plants absorb mate- 
rials from the soil in different proportions? 

On the basis of their nutrition, plants may be grouped into 
(1) the green plants, arid (2) the non-green plants. The green 
plants, which include all common crop plants, absorb mineral salts, 
Water, and carbon dioxide, and from them make the plant foods. 
The non-green plants, such as bacteria, yeast, molds, mildews, 
rusts, smuts, and mushrooms, subsist on living or dead plants or 
animals and derive foods from them directly. But the foods 
the materials which nourish the plant body are the same for both 
plants and animals. 

Considering green plants the common plants of garden, field, 
orchard, and forest we can say that the source of the chemical 
elements which make up their bodies is derived from the air and 
soil. From the air they derive carbon and oxygen; from the soil, 
oxygen and all other chemical elements which compose the plant. 

The principal substances found in plants belong to the chemical 
groups known as carbohydrates, fats, and proteins. The principal 
carbohydrates are starch, sugars, and cellulose. Starch is proba- 
bly the most important storage product in all plants. It is found 
in fruits, seeds, leaves, roots, and stems in fact, there is scarcely 
a part of the plant that does not contain some starch. In most 
leaves, starch accumulates during the daytime, and at night is 
changed to sugar, r which moves in solution to various parts of the 
plant where it is stored in some form. Starch occurs in the vas- 
cular rays of woody stems and roots, and in other tissues of these 



organs. Starch is accumulated in large quantities in many seeds, 
such as wheat, oats, barley, beans; in certain roots such as pars- 
nips; and in certain special types of storage stems such as potato 
tubers. Starch is used by the plant as a stored or reserve food. 
When needed for growth, it is digested, and the digested products 
move to growing points. 

Make a list of the principal starch-storing food plants. 
There are many dif- 
ferent kinds of sugars 
in plants. The princi- 
pal ones are grape sugar 
(glucose), cane sugar 
(sucrose), and fruit sugar 
(fructose). Glucose is 
considered to be the 
immediate product of 
photosynthesis, that is, 
of the process by which 
water and carbon diox- 
ide are converted into 
this product. Glucose 
is the usual form in 
which carbohydrates 
move from one part of 
the plant to another 
part. It is found in 
the conducting tubes, 

FIG. 18. Section through the tuber of Irish 
potato. The flesh of the tuber consists chiefly 
of large, thin-walled starch-storing cells, in 
which are scattered bundles of conducting 
tissue. A ring of vascular bundles is visible, 
and at the right-lower corner is an "eye." 

and in the sap of most 
cells. It appears to be 
the foundation material 
used in the synthesis 
of most other plant sub- 

stances. Sucrose or cane 

sugar is particularly abundant in the root of sugar beet and 
in the stems of sugar cane. Fructose or fruit sugar is particularly 
common in fruits. 

Cellulose is a carbohydrate which enters into the structure of 
cell walls. It is structural material. 



The fats or oils occur chiefly in seeds. For example, in the seed 
of the cotton plant there is much reserve oil. It is this oil which 

the seed uses as a food supply 
during the germination process. 
Make a list of plants which are 
the source of commercial oils. 

Proteins are reserve foods 
found principally in seeds, such as 
beans, peas, and cereals. Proto- 
plasm, itself, is rich in proteins. 
In addition to the carbo- 
hydrates, fats, and proteins just 
mentioned, numerous other sub- 
stances are found in plants. For 
example, there are the various 
plant pigments, resins, gums, 
alkaloids, organic acids, and es- 
sential oils. 

As regards the chemical composition of the plant, we may sum- 
marize as follows : The framework of the plant that is, the walls 

FIG. 19. Cross-section of a sugar 
beet root. Observe the rings of 
growth, all of which are produced in 
one season. Most of the cells of the 
root are rich in sucrose (cane sugar). 

&& r ' ' --- n.- i; ^a&afa 


=^. i inner 
=^ I integument 

FIG. 20. Microscopic section of wheat grain. The aleurone layer and the 

starchy endosperm are rich in stored food; the other layers constitute the 

"bran." (From Robbins, in Botany of Crop Plants.) 


of all cells is chiefly cellulose; protoplasm itself is largely a mix- 
ture of various proteins and fatty substances in water; the chief 
products stored in cells are sugars, starch, and proteins; and, in 
addition to these more common materials, there are gums, resins, 
alkaloids, acids, various pigments, and essential oils. The sap 
of a plant is chiefly water, carrying in solution salts derived from 
the soil, and various other substances manufactured by the plant. 
The use of plant materials by man and animals. It is well 
recognized that all animal life on the earth, including man himself, 
is dependent upon plants for its very existence. Green plants 
are the only agencies by which the inexhaustible supply of solar 
energy is caught and held, and transformed into foods, for plants 
and animals alike. We are familiar with the fact that plants 
furnish us with food, clothing, shelter, and numerous drugs; that 
plants are used to beautify our homes and landscapes; that they 
supply feed for livestock; and are used in the manufacture of 
scores of commercial products. In Unit X there will be a more 
extended discussion of plants of economic importance to man. 


1. What do we mean by saying that the cell is the unit of structure of the 

2. Enumerate the different things that protoplasm of plants can do. 

3. Justify the statement that protoplasm is the most wonderful substance 
in the world. 

4. Why do complex plants need different kinds of tissues and organs? 

5. Explain why plants which live on the land have more highly developed 
tissues and organs than plants which live in the water. 

6. Give the main characteristics of each of the four great groups of plants. 

7. Upon which of the four great groups does man depend chiefly for his 
food supply? 

8. Name as many farm crops as you can in which the root is useful to man; 
the leaf; the stem; the flower; the seed; the fruit. 

9. Which of the four groups of plants is of least importance to man? 
30. Of what importance to plants is the food stored in the seed or root? 


Plants Useful to Man, by W. W. ROBBINS and FRANCIS RAMALEY, pub- 
lished by P. Blakiston's Son and Company, Philadelphia, 1933. A well- 
illustrated book (241 figures) of 428 pages giving a discussion of the principal 


food plants of the world, spices, beverage plants, medicinal plants, and indus- 
trial products of vegetable origin. "It furnishes a background of knowledge 
of the world's commercial plant products, both for students of botany and for 
those whose interests are in the fields of geography, economics, and agri- 

The Growth of Biology, by WILLIAM A. LOCY, published by Henry Holt 
and Company, 1925. "The growth of our knowledge of living organisms is 
a part of the larger story of human progress; the struggles and triumphs of 
the human spirit. In a history of any science it is not sufficient to give an 
impersonal account of the discoveries as coming in a certain sequence the 
human element is involved as an essential part of the story. " Among other 
things, it includes an interesting account of the discovery of the plant cell, 
the development of our knowledge of protoplasm, and the structure of plants. 
481 pages and 140 illustrations. 

Geography of the World's Agriculture, by V. C. FINCH and O. E. BAKER, 
published by the United States Department of Agriculture, Washington, D. C. 
1917. 149 pages, 206 figures. "The purpose of this study is to show the 
geographic origin of the world's supply of food and of other important agri- 
cultural products and to indicate briefly the climatic, soil and economic 
conditions that account for the distribution of the crops and livestock of the 
world/' There are numerous maps and figures, 


If we classify all living things on a food basis, we will have 
two groups. In one group there will be all green plants; in the 
other group, all animals, including man, and all plants such as 
bacteria, rusts, smuts, toadstools, etc., which lack a green color. 
The first group is independent; the second absolutely dependent 
upon the first for its food. By " independent " we mean possess- 
ing the ability to manufacture food out of such materials as the 
plant absorbs from the soil and air. We, as animals, just like the 
non-green plants, do not possess organs which enable us to take 
such simple materials as carbon dioxide, water, and mineral salts 
and make them into foods that is, into carbohydrates, fats, and 
proteins. Only green plants have this power; they alone make 
the food, not only for themselves but also for every other living 
creature. The important fact for us to understand, the fact which 
is emphasized in this chapter, is that the food laboratory of the 
world is to be found in the green plant. 

Foods the carbohydrates, fats, and proteins possess energy. 
When foods are oxidized, either in the bodies of plants or animals, 
that is, broken down into simpler substances, they liberate energy, 
and this energy is used by the living body to do work. In the 
building of these foods by green plants, energy is stored up. What 
is the source of this energy? We well know that energy can not 
be created or destroyed. But it can be changed from one form 
into another. For example, sunlight is a form of energy. We 
call it solar energy. The green plant has the unique power of 
converting solar energy into the energy represented in foods, 
which is chemical energy. This means, in short, that all life on 
the earth depends upon sunlight. The student will gain much by 
reading from Chapter I of Spoehr's Photosynthesis, published by the 
Chemical Catalog Company, New York. This chapter deals with 



the origin of organic matter and the cosmical function of green 

Sunlight has been called the " prime-mover of civilization/' 
Discuss the significance of this statement. 

Some animals, like fleas, lice, and ticks, for example, gain all 
their food from animals. Show how, in the last analysis, they are 
dependent upon green plants for their food, and finally upon sun- 
light for their energy. 

Problem 1. What is the nature of plant foods? 

We are accustomed to speak of the mineral salts containing 
such essential elements as nitrogen, sulphur, phosporus, calcium 
magnesium, potassium, and iron as the "food" of plants. This 
statement is not strictly true. A " food " for plant or animal 
is a substance that can be incorporated directly by the living cells 
and used as a source of energy or in making new plant substances. 
The mineral salts, as such, cannot nourish the living cells any more 
than can nails and tacks, if taken into the human stomach. The 
mineral salts are used by the plant not as a "food "but as a 
raw material from which foods are made. 

The foods the substances which are capable of furnishing 
energy or of building tissue for all plants and animals are identi- 
cal. The chief foods of plants and animals are substances known 
chemically as carbohydrates, fats, and proteins. Well-known 
carbohydrates are sugars and starches. Fats occur in the liquid 
state (" oils ") or solid state (" fats "). Proteins are the most 
complex, chemically, of all foods. Carbohydrates contain but 
three chemical elements; carbon, hydrogen, and oxygen; fats also 
contain the elements carbon, hydrogen, and oxygen, but in much 
different proportion from that in which they occur in carbohy- 
drates; and proteins possess, in addition to carbon, hydrogen, and 
oxygen, such elements as nitrogen, phosphorus, sulphur, and 

Exercise 14. Lists of food-storing plants. Make lists of plants which 
manufacture and store large amounts of each of the following foods : starch, 
sugar, oils, proteins. 

Exercise 16. How to test for starch in plants. Cut a thin section of the 
potato tuber, place this on a microscope slide, and cover with a drop of iodine 


in potassium iodide (0.3 gram iodine, 1.5 grams potassium iodide, 100 cc. 
water). Discuss results. 

Exercise 16. How to test for sugars in plants. The juice of plant tissues 
may be pressed out and tested for sugar as follows Prepare Fehling's solution 
thus: In bottle A dissolve 6.9 grams of copper sulphate crystals in 100 cc. 
water; in bottle B dissolve 34 grams of Rochelle salt (potassium sodium 
tartrate) and 12 grams of sodium hydroxide in 100 cc. water. Keep the two 
solutions A and B separate. To the plant juice to be tested add equal quan- 
tities of A and B, and heat slowly in a test tube over a Bunsen burner to 
boiling. A precipitation of red cuprous oxide crystals indicates the presence 
of glucose or other reducing sugar. Cane sugar (sucrose) does not give the 
Fehling's test. However, by treating cane sugar solution with a few drops ol 
concentrated sulphuric acid, it is changed to glucose and fructose, both of 
which give the characteristic Fehling's test. 

Exercise 17. How to test for fats in plants. Fat bodies and membranes 
containing fats give a characteristic red or orange color when treated with a 
solution of Sudan III (1 gram Sudan III crystals in 200 cc. 70 per cent alcohol). 

Exercise 18. How to test for proteins in plants. Grind a soaked bean 
in a mortar. Place this in a test tube and add a little water. Add 5 cc. of 
nitric acid and heat slowly to boiling. Allow the solution to cool, pour off 
the acid, and add 10 cc. of ammonium hydroxide to the bean material in the 
tube. A deep orange color indicates the presence of protein. Test other 
plant materials for protein. 

Is there any difference between the foods of animals and those of plants? 

What is meant when we say that a plant is " independent " or " depend- 
ent"? Name five independent plants and five dependent plants. 

Suggested activity. Consult dietary charts and make a collection of foods 
produced by plants which are rich in carbohydrates, another of those contain- 
ing large amounts of fats, and a third of those having much protein. 

Problem 2. What are the raw materials used by green plants 
in the manufacture of food? 

We have learned that the principal foods of plants are carbo- 
hydrates, fats, and proteins. Also, we have learned that only 
green plants are capable of manufacturing foods. It has been 
found by careful experimentation that the building of foods by 
green plants proceeds by rather definite stages. The initial 
process appears to be the building of a simple sugar, known as 
glucose. The fact is, this sugar is the foundation material upon 
which the more complex foods, such as fats and proteins, are 
built. Without this simple carbohydrate, manufactured in green 
cells, as a " starter/ ' the more complex foods could not be formed. 


The raw materials in the manufacture of glucose are carbon 
dioxide and water. Carbon dioxide comes from the atmosphere, 
water from the soil. The process of manufacturing glucose from 
carbon dioxide and water, in the presence of light, is called 

It was for a long time believed that humus (decomposed plant 
and animal material) in the soil was the source of carbon for 
plants. But if a plant is grown in pure sand, free from humus, it 
will increase in the weight of carbon. Moreover, it has been found 
by experiment that green plants placed in an atmosphere from 
which every trace of carbon dioxide has been removed soon 
cease to grow, and that green plants cultivated in a soil from 
which every trace of compounds containing carbon is removed 
thrive perfectly. It is concluded that carbon dioxide is essential 
to a green plant and that the source of carbon in the plant is the 
carbon dioxide gas of the atmosphere. It is useless to try " feed- 
ing " plants carbon by applying fertilizers to the soil. Carbon 
dioxide enters the plant through small pores in the leaf surfaces; 
water enters through the root hairs. These two simple chemical 
compounds or raw materials are brought together in those cells of 
the plant, chiefly leaf cells, which contain a green coloring matter 

After the plant has manufactured glucose sugar in its green 
tissue, this sugar now becomes building material or basis for other 
foods. Part of it is converted into cellulose for the formation of 
the walls of new cells, and for the thickening of old walls of living 
cells; a portion of it is used in the production of oil; part of it is 
employed, together with nitrogen, sulphur, phosphorus, and other 
compounds of simple character, to form proteins, and part of it is 
oxidized in the process of respiration. The chemical elements 
nitrogen, sulphur, phosphorus, potassium, magnesium, calcium, and 
iron, all of which are indispensable, either directly or indirectly, in 
the building of certain plant foods, all come from the soil, occurring 
in the soil and entering the plant in the form of a salt. The chief 
salts supplying these elements are the nitrates, the phosphates, 
and sulphates. 

Suggested activity. Prepare a paper on " mineral fertilizers." Name the 
chief chemical substances found in those offered by local dealers, and show 


what results may be expected from the use of each, as large top growth, a 
green lawn, or large yield of grain. 

Summarizing: the raw materials used by green plants in the 
manufacture of food are (1) carbon dioxide, (2) water, and (3) 
mineral salts. It will be noticed that all these substances belong 
to the chemical group known as inorganic. In other words, the 
taw materials out of which green plants make food are inorganic 
compounds. The foods belong to a group of chemicals known 
as organic. These are compounds which for the most part com- 
pose the bodies of plants and animals. So, the green plant con" 
verts inorganic materials into organic a process peculiar to green 
plants alone. Also, note that, of the raw materials, water and 
mineral salts come from the soil and carbon dioxide from the air. 

Before describing in more detail the processes in the building 
of foods by plants, let us find out how the raw materials carbon 
dioxide, water and mineral salts enter the plant, and how they 
move to the parts of the plant where they are used. 

Problem 3. How do raw materials enter the plant? 

Water and mineral salts are raw materials which come from 
the soil and enter the plant through the roots. First of all, let us 
find out the important facts about the different kinds of roots and 
their functions. 

Kinds of root systems. There is much variation in the form, 
the spread, and the depth of the root systems of plants. Two 
common types of root systems are recognized. The taproot 
system is well illustrated in such plants as the beet (Fig. 21), rad- 
ish, turnip, parsnip, dandelion, maple, and pine. In this there is 
one main root, which grows almost directly downward, giving off 
numerous branch roots. The fibrous root system is seen in its 
typical form in such plants as wheat, oats, corn, and other cereals. 
In this, one can not distinguish a main root, but there is a great 
number of relatively small roots of about the same size which 
form a network. In still another form of root system, com- 
mon to many of our fruit trees, there are several large roots of 
about equal size which anchor the plant in the ground and which 
give off numerous finer branch roots. 



If soil conditions are favorable, the taproot by its direct down- 
ward growth is able to penetrate the deeper layers of soil; for this 
reason it is adapted to dry regions. The taproot of alfalfa, for 
example, may extend to a depth of 10 to 12 feet, or even more. 

The fibrous root system, on the other hand, is usually shallow. 
Fibrous-rooted plants are employed as soil binders on ditch banks. 

^ ~f ; ' x -,- ,.,,. 

' l- /r *"..:> ',-* 

f fvi: 

FIG. 21. Tap-root system of young sugar beet. (From Robbing, in Botany 

of Crop Plants.) 

steep hillsides, and in other situations where the soil is likely to be 
moved away by rain or wind. In the reclamation of eroded soils, 
fibrous-rooted plants are used extensively. 

Exercise 19. Field study of root systems. Study in the field and report 
on the root systems of a number of common plants. Along the steep banks 
of streams or where erosion is rapid, one may be able to trace the root systems 


of trees and shrubs. (Refer to Root Development of Field Crops by John E. 
Weaver, McGraw-Hill Book Company, New York.) 

Factors which influence the growth and character of the root 
system. The depth and spread of the roots, although characteris- 
tic of the kind of plant, are nevertheless influenced by environ- 
mental conditions, chiefly the water content of the soil, the air 
supply, and the available mineral nutrients. These in turn are 
modified by tillage, fertilizers, crop rotation, irrigation, and 

In most of our fruit plants the root systems extend as deeply 
in the soil as the air supply of the soil will permit, providing there 
is sufficient moisture. It is undoubtedly true that air supply of the 
soil often limits root development. It should be repeated here 
that all living cells must have a supply of oxygen in order to live; 
the root hairs and other active cells of the roots must secure most 
of their oxygen from the soil air which immediately surrounds 
them; sufficient oxygen is probably not conducted long distances 
through the tissues of the plant from above to below ground. The 
absence or scarcity of root hairs in very wet soil, and in water, can 
probably be attributed to poor oxygen supply. In a water-soaked 
soil, the air spaces are filled with water. 

Excessive irrigation may produce an actual decrease in the 
yield of a crop, chiefly because it produces a soil condition in 
which aeration is faulty. It has been said that the best irrigation 
practice involves the most effective compromise between too 
much water and too little air. Why do florists use porous earthen- 
ware pots with a hole in the bottom? 

Soil fertility influences the root development of a plant. It has 
been found that " crops grown in soil of high fertility have roots 
that are shorter, more branched, and more compact than those in 
similar but less fertile soil." Moreover, it has been shown that, 
where roots in their growth come in contact with a fertilized layer 
of soil, they not only are more branched, but they also are slow in 
penetrating the soil below. 

Transplanting and root pruning encourage the development 
of branch roots. In transplanting, many root tips are injured 
and, as a result, branch roots are stimulated to develop. Thus, a 
compact root system is formed. 



The extent of root systems. The extent of roots is often much 
greater than that of the top growth. This is true of many native 

species of plants and also of a 
number of cultivated plants. For 
example, in the sugar beet, the 
main root may extend to a depth 
of 5, 6, or 7 feet, and give rise to 
numerous branches which spread 
laterally several feet. Corn has a 
root system which may occupy as 
much as 200 cubic feet of soil. 
Weaver points out that a corn 
plant in the eight-leaf stage has 
from 8,000 to 10,000 lateral roots 
from the 15 to 23 main roots. A 
corn plant 5 weeks old, grown in 
a fertile, moist soil, developed a 
root system the absorbing area of 
which (excluding the root hairs) 
was 1.2 times as great as the area 
of the top, and in a dry soil 2.2 
as great as the area of the top. 
Weaver also found that the root 
area of Turkey Red winter wheat 
exceeded that of the top by 10 to 35 per cent. The roots of com- 

FIG. 22. Seedling of corn. The 
primary or temporary roots (C), 
adventitious or permanent roots 
(B), and the stem structure (A) 
which may be long or short de- 
pending upon the depth at which 
the seed was planted. 

FIG. 23. The sweet potato is a fleshy storage root. 

mon asparagus may extend to a depth of 8 or 9 feet and as far 
laterally, occupying the soil very completely. The roots of a 


7-year-old apple tree were known to spread horizontally more 
than 12 feet and to a depth of 9 feet. 

Kinds of roots and their functions. 

Exercise 20. Primary and secondary roots. Examine the roots of 
radish and wheat seedlings of various ages. Note that in each one there is, 
first of all, a primary root which develops from the radicle or rudimentary root 
of the embryo of the seed from which the plant grew. And in each there are 
small secondary roots, branches of the primary root. Compare older seed- 
lings of radish and wheat. Does the primary root of the radish become the 
principal root throughout the life of the plant? In older seedlings of wheat, 
observe that numerous fibrous roots arise from the base of the young stem. 
These come to form the permanent root system of the wheat plant, whereas 
the primary roots (those from the embryo) are but temporary. The perma- 
nent roots of wheat are said to be adventitious (Fig. 22). 

Exercise 21. Adventitious roots. Make stem cuttings of willow, gera- 
nium, or begonia; place in moist sand, and observe the position and nature of 
the roots which arise at the cut surface. These are adventitious roots. 

Exercise 22. Prop roots. Observe the prop roots of corn, the clinging 
aerial roots of ivy, and the aerial roots of orchids. 

Exercise 23. Root crops. Make a list of plants which are grown pri- 
marily for their roots. 


By way of review of the studies thus far made of roots, write out answers 
to the following: 

1. Name two kinds of roots as to origin, giving examples of each. 2. Name 
three kinds of roots as to the medium in which they grow, with examples of 
each. 3. Name four kinds of roots as to form, with examples of each. 
4. Give the functions of roots, and after each function name a plant whose root 
performs that function. 5. From what part of the embryo does the primary 
root grow? 6. What is the usual direction of growth of primary root? Of 
secondary roots? Give advantage of each direction to the plant. 7. Name 
plants whose roots live but one year, others whose roots live two years, and 
still others whose roots live many years. 

Structure of roots, and their adaptation to absorption of raw 
materials. Now that we have in mind the principal kinds of roots 
and root systems, let us inquire into their structure and how they 
are adapted to the intake of raw materials from the soil. We 
have learned that anchorage is one of the most important func- 
tions of the roots; also that many roots store food, and that a few 
may be used to propagate the plant. A primary root function is 



Exercise 24. Root structure external features. Germinate seeds of 
red top or blue grass by dropping the dry seeds on the surface of water in a 
large flat dish and keeping at room temperature for three to five days. Study 
the primary roots in water in a watch glass when they are % to % inch long. 
Place the watch glass on the stage of a compound microscope and examine with 
low power to see the principal structures. Note the root cap, the central 
conducting cylinder surrounded by a sheath, the cortex, and the root hairs. 
Make a diagrammatic sketch of the root, and label all parts. 

Exercise 25. Root structure in- 
ternal features. Mount young roots 
as above in water on a microscope 
slide, and cover with cover-slip. Ex- 
amine with low power of compound 
microscope. Identify the parts of the 
root as observed in the preceding ex- 
ercise. While watching, gently press 
on the cover-slip so as to crush the 

FIG. 24. A young root tip cut 
lengthwise. Observe the root 
cap, the central conducting cyl- 
inder, and the cortex. 

FIG. 25. A portion of a root showing 

stages in the development of a root 

hair. A root hair is an outgrowth of 

an epidermal cell. 

root. In the region of the central cylinder one should be able to see the 
vessels. These are the structures which conduct water and mineral salts 
up through the roots. 

Exercise 26. Root structure as seen in cross-section. Examine prepared 
cross-sections of young roots. Observe the following zones or regions: (a) the 
epidermis with its root hairs; (&) the cortex, a sheath surrounding the (c) cen- 
tral cylinder, and within the central cylinder, strands of phloem and xylem. 


Phloem possesses the structural elements which conduct foods, whereas xylem 
has the structural elements which conduct water and mineral salts. 

Root hairs (Fig. 25). We are familiar with the fact that, in 
ordinary land plants, the very important work of absorbing water 
and mineral nutrients from the soil is confined to the roots. But 
not all root surfaces can absorb. On the slender, thread-like roots 
are innumerable outgrowths, the root hairs, and these are the prin- 
cipal absorbing organs. In some plants it is almost impossible 
to see them without a magnifying lens. However, they may stand 
so thickly on the fine roots as to form a white fuzzy growth. 
The younger parts of the very fine rootlets which have not become 
corked over absorb to a limited extent; but in the majority of 
plants by far the greatest amount of absorption is carried on by 
the root hairs. Even the fine rootlets soon become covered with a 
corky layer which is impervious to water. 

Transplanting destroys root hairs. If a plant is transplanted 
or disturbed in cultivation or at thinning, it wilts. We may see 
no apparent injury to the roots. However, on closer examination 
we observe that the root hairs have had their connection with the 
soil particles broken, and that many of the finest rootlets bearing 
root hairs are destroyed. The plant does not recover until new 
root hairs are developed. 

It is well known that it is dangerous to transplant young trees, 
shrubs, or vines when they are clothed with leaves and are actively 
growing. In this condition the plant demands more water than 
when in a dormant state. If the fine rootlets and root hairs are 
destroyed, as is largely unavoidable during transplanting, the 
demands for water are greater than the roots can supply. Exces- 
sive watering will not make up for the loss of roots and root hairs 
by trees, shrubs, and vines transplanted during the growing sea- 
son. The roots may be surrounded by nearly saturated soil, but 
in the absence of root hairs, they can not absorb adequate amounts 
of water. In transplanting it is the usual practice to reduce the 
water-losing area (leaf surface) by pruning. 

Exercise 27. Relation of root hairs to soil particles. Carefully remove 
seedlings growing in fine sand. Observe the relation of root hairs to soil 
particles. (Fig. 26.) 



The soft, delicate root hairs, in growing, wrap themselves about 
and spread over the soil particles coming in very close contact 
with them. Thus, they expose a maximum surface to the parti- 
cles, from which they absorb both water and mineral salts. In a 
soil that is well drained and contains the 
proper amount of moisture, the water occurs 
as very thin films and as wedge-shaped 
masses at the points of contact of the soil 
particles, and in this water the various min- 
eral salts are dissolved. Consequently, by 
growing about the particles the roots place 
themselves in the best position for getting 
water and salts. 

Root hairs increase the absorbing sur- 
face of a plant. A plant with hundreds of 
thousands of root hairs on its fine rootlets has 
a tremendous total absorbing surface. The 
fine rootlets alone, without the root hairs, 
would have a relatively small total surface. 
It has been computed that a corn plant with 
root hairs has an absorbing surface of about 
6 times that of one from which the root 
hairs have been removed. 

Duration of root hairs. Root hairs have 
a very short life. As the rootlet upon which 
they grow pushes its way through the soil by 
growth at the tip, new root hairs are contin- 
ually formed just behind the tip, and the 
oldest root hairs farther back are constantly 
dying off. Any one individual root hair lives 
only a day or so. Thus there is a constant 
new supply of rapidly growing root hairs. 
Moreover, the roots are continually exploring new soil 

Exercise 28. The structure of the root hairs. Study the structure of 
root hairs with a compound microscope. Note that each root hair is a tube- 
shaped body, growing out from an epidermal cell of the rootlet. In fact, 
each root hair is a single plant cell with the power to carry on the various life 

FIG. 26. Wheat 
seedling showing soil 
particles clinging to 
root hairs ; note that 
the root cap is free 
of root hairs. 
(From Robbins, in 
Botany of Crop 


processes. It is a living structure, The living material and the cell sap are 
surrounded by a non-living wall. The wall is lined on the inside with a very 
thin membrane of living material, the protoplasmic membrane. This mem- 
brane is an extremely important part of the cell, as we shall see later. Make a 
drawing of a root hair and several bordering cells. 

Substances from the soil enter the plant chiefly through the 
root hairs. It should be emphasized that almost all materials 
which enter the plant from the soil must pass through the wall 
and living membrane-lining of the root hair. From the root hair, 
the absorbed materials are passed on to the other root cells, and 
from them, into conducting tubes and thence to various parts of 
the plant. 

It used to be thought that plants " suck up " tiny particles 
of soil, and " feed " upon them. It is now known that the plant 
is incapable of absorbing solid particles. All materials taken in 
by the plant from the soil must be in a liquid state, that is, they 
must be in solution. Otherwise, they cannot pass, or diffuse, 
through the root-hair wall and the living membrane which lines it. 

Give the structural features of a root which fit it to the process 
of absorption. What is the function of the root cap? Compare 
soil roots with roots of water plants, and also with the air roots of 
an orchid. Explain the differences. How do root hairs differ from 
rootlets in structure and in place of origin? Do root hairs become 

The process of absorption. The principal point to kedp in 
mind in connection with absorption by roots is that all substances 
which enter the root hairs from the soil must be in solution. 
Water is the solvent. The second important point is that all 
substances which pass from the soil to the inside of the root hair 
must pass through the cell wall of the root hair and also the living, 
protoplasmic membrane which lines the wall. Several simple 
experiments should first be performed by the student so that he 
will better understand the process of absorption. 

Exercise 29. Diffusion. In the bottom of a beaker or glass tumbler of 
water place a few crystals of copper sulphate (blue vitriol). If the vessel is not 
disturbed, the copper sulphate goes gradually into solution. At first the 
solution is a deeper blue in the neighborhood of the crystals, the color being 
less and less blue as the distance from the crystals increases. This indicates 


that particles (molecules) of copper sulphate are gradually moving out into 
the water. After a long time, the whole solution becomes a uniform blue 
color, indicating the equal distribution of copper sulphate molecules through- 
out. In the solution there are two components, water and copper sulphate. 
Water is the solvent, copper sulphate the solute. In this experiment the 
movement of copper sulphate molecules into the water from a region where 
they are in greater concentration to one where they are in less concentra- 
tion, and the movement of water molecules into the copper sulphate from a 
region where they aie in greater concentration to one where they are in less 
concentration, illustrate diffusion. Keep in mind that diffusion plays a most 
important part in the absorption of materials by the roots. 

Exercise 30. Diffusion through membranes. In the preceding experi- 
ment, the two substances, water and copper sulphate, were not separated 
by any kind of a membrane. Now let us devise an experiment which resem- 
bled, in part at least, the situation represented in the root hair. Recall that the 
sap in the root hair is separated from the soil solution by two membranes: 
(1) the cell wall, composed of non-living matter called cellulose; and (2) the 
living protoplasmic membrane. The two membranes have very significant 
differences in their behavior. 

With a carpenter's 1-inch bit, drill a smooth hole down into the middle of a 
fresh carrot, being careful that the bit does not cut through the surface at any 
point. Connect a one-hole No. 6 rubber stopper to the end of a 5-foot length 
of glass tubing of small diameter. Pour corn syrup into the carrot, and fit the 
stopper into the hole so that the syrup extends into the glass tubing to a point 
above the stopper. Support the carrot in a jar of water so that the surface of 
the water is on a level with the surface of the syrup in the tube. After a time 
you will observe that the liquid in the tube has risen. We are forced to the 
conclusion that more material passed into the sugar solution than passed out 
of it. Both kinds of membranes are present in the shell of carrot surrounding 
the sugar solution. The living protoplasmic membranes of the carrot are 
semi-permeable. That is, they allow some substances to pass through them 
more readily than others. In this case the sugar molecules pass through the 
membrane only slowly, if at, all, and since there is a relatively greater concen- 
tration of water molecules on the outside than in the syrup on the inside the 
more rapid diffusion of water is toward the inside. 

The protoplasmic membrane of a root hair is a semi-permeable 
membrane. The cell wall is wholly permeable, that is, does not 
inhibit the movements of any kind of molecules through it. Con- 
sequently, in the movement of materials into the root hair, the cell 
wall can be disregarded. The root-hair sap is a solution containing 
not only sugar and other organic compounds, but also a number of 
different salts, all in solution in water. The soil solution contains 
many different mineral salts in aqueous solution. These two 


solutions that of the root-hair sap and that of the soil are sepa- 
rated by a semi-permeable membrane, the protoplasmic membrane, 
which does not allow all substances to move through it with equal 
ease and speed. 

Under natural conditions, the cell sap of the root hair is 
always more concentrated than the soil solution; that is, the water 
molecules are in less concentration inside the root hair than out- 
side. We learned from the foregoing experiment that water 
passed from the region where the water molecules are more con- 
centrated into a region where they are less concentrated. Apply 
this to the movement of water from the soil into the root hair. 

Exercise 31. Plasmolysis. This experiment throws further light on the 
movement of substances through semi-permeable membranes. Immerse 
whole fresh leaves of the water plant Elodea into a strong solution of ordinary 
table salt or sugar. The solution can be added to the leaf on a microscope 
slide, covered with cover-slip, and observed with the high-power compound 
microscope. After a while it will be noticed that the contents of the leaf 
cells are shrinking, and pulling away from the wall. This shrinking, known 
as plasmolysis, is due to the withdrawal of water from the cells by the solution 
outside. Water continues to move out of the cell just as long as the solution 
outside is more concentrated than that in the cell. In the plasmolyzed cell 
what solution is between the cell wall and the protoplasmic membrane? 

Exercise 32. Turgidity of cells. The swollen state of cells is called tur- 
gidity. We know that the crispness of the leaves and the rigidity of the young 
parts of plants is due to the turgid condition of the many hundreds of 
thousands of individual cells which compose it. We can understand this 
behavior better if we consider the following exercise. When slices of potato 
are placed in water, they remain rigid. All cells are well rilled with water, and 
as a result are swollen. The combined effect is a turgid slice. If, on the other 
hand, the slices of potato are placed in a strong salt or sugar solution, they 
soon become limp and flexible; all cells lose water; it passes from them into 
the strong solution outside where water molecules are in less concentration. 
As a result, each cell is like a partially deflated automobile tire, and the whole 
slice is of the same character. It is not so much the nature of the substance as 
its concentration which determines the movement of water in or out of the cell. 
For example if the slices of potato are put in a salt or sugar solution, the strength 
(concentration) of which is less than that of the sap in the potato cells, they 
will remain fresh* instead of losing water they will absorb it. The cells of the 
slices will lose wacer only when the strength of the solution in which they are 
immersed is greater than that of the solution in them, that is, when the water 
molecules are in greater concentration inside the cells. Water tends to move 
from a solution of low concentration to one of higher concentration. In most 


agricultural lands, the soil solution has a lower concentration than has the 
solutions in the plant. Consequently, the movement of water is from the 
soil to the plant. 

The normal, active root hair is swollen and distended like an 
automobile tire inflated with air. It is filled with water, various 
mineral salts in solution, and other substances, all of which render 
it swollen. It does its best work only in this distended condition. 
If, through lack of water, the plant wilts, the root hairs, as well 
as other cells of the plants, collapse to a certain extent, and in this 
state, they do not perform their work properly. Any part of a 
plant in a wilted condition is incapable of making growth. It may 
remain alive and be able to recover if given water, but it will not 
grow. Withholding water from a plant to the extent of causing 
prolonged wilting cannot fail to retard its growth. 

Conditions which influence the rate of absorption. Several 
conditions have a marked influence upon the rate of water absorp- 
tion by root hairs. In the first place, when the amount of moisture 
in the soil reaches a low point, the rate at which it is absorbed by 
the roots is diminished. There are forces in the soil which hold the 
water, and unless the forces of absorption exceed these soil forces, 
water will not be taken in by the roots. 

The temperature of the soil is another important factor deter- 
mining the rate of water intake. The rate is decreased by lowering 
the temperature, and most plants cease to absorb at or slightly 
above freezing. Place a small potted plant in a dish of ice water, 
and another in water of room temperature. After a time note 
results. Explain. 

The air supply of the soil also influences the absorption rate. 
Root hairs do their best work only when they are well supplied 
with air. In soil that is compacted, or so wet that air is excluded 
from the soil spaces, absorption is slowed down. 

The strength or concentration of the soil solution is a very 
important condition determining the rate at which water from 
the soil enters the plant. Normally, the solution in the root hair is 
of greater concentration than that in the soil. Under this con- 
dition, water moves into the hair through the living membrane 
which separates the two solutions. If, on the other hand, the soil 
solution should become more highly concentrated than that in the 


root hair, water would move from the root hair to the soil, and a 
wilting of the root and entire plant would follow. In an alkali 
soil, the soil solution surrounding t! e root hairs is often quite con- 
centrated ; at times its strength may approach that of the sap of the 
root hairs; under these conditions, water movement inward is slow, 
and the plant finds it difficult to take in as much water from the 
soil as it loses through the leaves to the air. Water continues to 
move inward as long as the concentration of the solution inside 
of the root hair exceeds that of the solution in the soil on the out- 
side. In other words, if the cell sap be denser than the soil solu- 
tion, that is, poorer in water, water will be absorbed from the soil 

It is well known that grass, weeds, and other plants can be 
killed by drenching the soil in which they are growing with a 
strong salt solution. In this case the plant is killed by a failure to 
absorb sufficient water or by an actual loss of water, rather than 
poisoned by the chemical. The root hairs, being surrounded by a 
soil solution which has a greater concentration than that of the 
root-hair sap, die because water is withdrawn from them. Soon 
the entire plant dies, for it continues to lose water through its 
leaves, while at the same time absorption of water is stopped. 

The rate of water absorption is also increased if the rate of 
water loss from the leaves is increased, providing there is an ample 
supply of soil water to draw upon. 

The movement inward of any particular salt appears to depend 
chiefly upon the available supply in the soil and the rate at which 
it is used in the plant. The essential elements which are absorbed 
by the plant are soon changed into new substances, and so their 
concentration in the plant is kept constantly less than that in the 
soil solution. 

Enumerate the factors which influence the rate of water absorp- 
tion by plants. 

Most substances in the root are not lost to the soil. It should 
be pointed out that, whereas substances pass from the soil through 
the root-hair membrane into the root, most substances in the 
root hairs are unable to pass back into the soil. Carbon dioxide, 
in the ordinary process of respiration of root cells, continually 
passes from the roots to the soil; and, under certain 


water also may be lost to the soil. It has been thought by some 
that under certain circumstances sugar in the sugar-beet root 
passed from the root into the soil. This is not the case. The 
percentage of sugar in the root may go up and down as a result 
of different amounts of water in the beet, but the actual ounces 
of sugar in the root do not become less, except under very unusual 
circumstances when the stored supply is called upon to make an 
abundance of new leaf growth. 

Summary. We may summarize our discussion of the nutrition 
of green plants thus far as follows: (1) The foods of plants, as well 
as those of animals, are organic, and chiefly carbohydrates, fats, 
and proteins. (2) These foods are built in the plant body, utilizing 
as raw materials carbon dioxide (from the air) and water and 
mineral salts (from the soil). (3) Water and mineral salts are 
absorbed from the soil by the roots, which are structures well 
adapted to the process of absorption. 


1. Define solute, solvent, solution. 

2. Why is it necessary to understand diffusion in order to study the process 
of absorption by roots? 

3. Explain why plants growing in cold bogs have difficulty in absorbing 

4. In what ways are alkali soils injurious to plants? 

5. What is meant by the "selective power" of root hairs? 

6. When slices of red garden beets are washed, and then placed in fresh 
cold water, the water does not become red. But when slices of such beets are 
boiled, the water soon becomes red. Explain. 

7. Why do young plants become limp when they wilt? 

8. Why does a dried prune swell when placed in water? 

9. Why is it possible to kill Canada thistles or other perennial weeds by 
the use of a liberal application of salt to the soil? 

10. Is it possible for a grower to injure or kill plants by applying to them 
too much mineral fertilizer? Explain. 

11. Explain why corn on land which has been flooded loses its green color 
and turns yellow. 

Types of leaves. We now have to consider the intake of carbon 
dioxide, and the plant structures (leaves) chiefly concerned in this 



Exercise 33. Types of leaves. The student should collect and bring into 
the laboratory a considerable variety of leaves and have them before him 
while reading the following paragraphs. A suggested list is as follows: oak, 
maple, rose, clover, pea, cherry, ash, locust, horse-chestnut, strawberry, corn, 
iris, lily-of-the-valley. 

All common leaves have a broad, thin blade, which is covered 
above and below by a thin, transparent epidermis (Fig. 28). The 
blade has two portions, the veins, and the soft green tissue known 
as mesophyll supported by the veins. 
Look up the literal meaning of the 
word mesophyll. Most leaves have 
a stalk or petiole which attaches the 
blade to the stem, and through 
which materials are conducted to 
and from the blade. Some leaves, 
such as those of grasses and lilies, 
have no petioles, and are said to 
be sessile (meaning, sitting). Some 
leaves also have a pair of leaf-like 
outgrowths at the leaf base where 
it joins the stem. These are known 
as stipules. 

The arrangement of veins in a 
leaf is spoken of as venation. This 
varies in different kinds of leaves. 
For example, in grasses, lilies, iris, 
etc., the veins that are easily seen 
run parallel to each other the full 
length of the blade. This is known 
as parallel venation. In most broad-leaved trees and shrubs, also 
in many herbs, the veins that are easily seen branch frequently 
and join again, so that they form a network. This is known as 
net venation. In many leaves with net venation there is a single 
midrib or primary vein, from which the smaller veins extend, 
somewhat like the divisions of a feather. Such leaves are said 
to be pinnately veined. In other plants there may be several 
principal veins spreading out from the upper end of the petiole. 
Such leaves are palmately veined. 

FIG. 27. Drawing of the blade 
and a portion of the petiole of a 
geranium leaf. Note the pal- 
mately-arranged veins which ra- 
diate out from the point of attach- 
ment of the petiole. Explain the 
roles of the petiole, the veins, 
and of the thin, expanded blade. 



Leaves vary greatly in form. A simple leaf (Fig. 27) has a 
single blade, either with or without a petiole, or stipules. A 
compound leaf has a leaf blade consisting of a number of separate 
leaf-like parts, called leaflets. How can you tell a leaf from a 

Leaves also vary in size, in shape, in nature of the margin, 
in kind of lobing, in number of leaflets, and in many other charac- 
ters. In fact, there are hundreds of terms descriptive of leaves. 

Exercise 34. Terms describing leaves. Using the following form, check 
off in the proper columns the terms descriptive of some 20 different kinds of 
leaves collected by you. 











Exercise 35. Types of leaves. Make accurate sketches of the following 
and label the parts: 

1. Simple pinnate leaf. 

2. Simple palmate leaf. 

3. Compound pinnate leaf. 

4. Compound palmate leaf. 

5. Parallel-veined leaf. 

Structure of leaves and their adaptation to absorption of carbon 
dioxide. We have seen that the leaf blade is a thin, flat structure 
which exposes a large amount of tissue to the gases of the atmos- 
phere. In this particular it would appear to be adapted to absorp- 
tion of carbon dioxide. But let us examine the microscopic struc- 
ture of the leaf and see if in any other ways it is adapted to this 

Exercise 36. Leaf structure. With binocular dissecting microscope, 
using 20 to 50 magnifications and strong illumination, both transmitted and 
reflected, examine both surfaces of a number of different kinds of leaves, 



noting form of epidermal cells, stomata, epidermal hairs, and other gross 
characters. The leaves of Tradescantia (Wandering Jew) have very large 

Exercise 37. Epidermis of leaves. Strip pieces of the epidermis from 
the lower and upper surfaces of some leaf, such as Tradescantia. Mount 
these with outer surface up, and examine with compound microscope. Are 
the stomata equally nu- 
merous on both surfaces? 
Are epidermal cells trans- 
parent? Make a drawing 
of several stomata and 4 
to 8 epidermal cells ad- 
joining. Label. See Fig. 

Thus we sec that, 
in addition to the great 
surface exposed by 
leaves, there are open- 
ings in the epidermis 
through which carbon 
dioxide and other 
gases may pass in. 
Now, lot us exam- 
ine the internal struc- 
ture of the leaf. For 
this purpose we need 

FIG. 28. At right, the upper epidermis of a 
geranium leaf is shown. A portion of the lower 
epidermis of the same leaf is shown at left. Both 
are made up of irregularly shaped cells. Bean- 
shaped cells, the guard cells, which fit together 
in such a way as to leave a slit, a stoma, be- 
tween each pair are seen at left. Why are 
stomata usually found only on the under side 
of the leaf? 

thin cross-sections of the leaf. 

Exercise 38. Anatomy of leaf. Examine with compound microscope 
prepared cross-section of somo loaf. Compare with Fig. 29. Note the (1) epi- 
dermal coverings, upper and lower; (2) the stomata with their guard cells; 

(3) the mass of green tissue between the epidermal layers, known as mesophyll ; 

(4) the veins; and (5) large air spaces between the mesophyll cells. Make a 
diagram of the leaf as seen in cross-section, labeling all parts. From your obser- 
vation of the slide, answer the following questions: (1) How do guard cells 
differ from other epidermal cells? (2) Is it possible for carbon dioxide which 
may come through stomata to diffuse throughout the leaf and come in contact 
with even the innermost cells? (3) Do you think that the leaf structure is such 
as to adapt it to absorption of carbon dioxide? Why? 

From our study of the leaf, we see that it has three kinds of 
tissue: (1) the epidermis, (2) the mesophyll, and (3) the veins. 
In the epidermis are stomata, each with two guard cells. These 



*7 Upper 




tuith Air 

Guard Cell 


FIG. 29. Cross-section of a typical leaf, showing its internal structure. 

change their shape, so that the stoma may be closed or open. 
Further on in our study of water loss from the plant we shall have 
more to say about stomatal movement. The mesophyll is com- 
posed of two kinds of tissue 
(Fig. 29): (1) the palisade 
parenchyma, and (2) the 
spongy parenchyma. Pali- 
sade parenchyma cells have 
their long axes at right angles 
to the leaf surface. There 
may be one to several layers. 
Spongy parenchyma usually 
forms somewhat more than 
half of the mesophyll. The 
cells are not elongated. Cer- 
tain parenchyma cells may 
form a single-layered sheath 
FIG. 30. This leaf has been treated with surrounding the veins. All 
a caustic soda solution to destroy the soft cells of the pa li sa( Je par- 
leaf tissue, thus making readily visible , ,, f ,, 
the framework of veins. These veins <*<*&**> a11 <*lls of the 
not only support the soft food-making spongy parenchyma, except 
tissue, but they carry materials. in certain plants those which 


surround the veins, and guard cells are abundantly supplied with 
chloroplasts. These possess a green pigment known as chloro- 
phyll. The chloroplasts, by virtue of their chlorophyll, play a most 
important part in the manufacture of food by the plant. 

The process of carbon dioxide intake. Only about 3 to 4 parts 
in 10,000 of the atmosphere is carbon dioxide. There appears to be 
nothing to prevent the diffusion of the gases of the atmosphere, 
including carbon dioxide, through the stomata, and into the inter- 
cellular spaces of the mesophyll tissue. We know that carbon 
dioxide gas is soluble in water, and that the walls of mesophyll 
cells are wet. Therefore, the surface film of water on the outside 
of mesophyll cells contains a quantity of carbon dioxide in solu- 
tion. In the diffusion of carbon dioxide, just like that of copper 
sulphate described in a previous exercise, the molecules move 
from a point where there are many of them to a point where there 
are fewer of them. Therefore, if carbon dioxide is being used up in 
the mesophyll cells, that which is in solution on the surface film 
of water readily diffuses through the wall and protoplasmic mem- 
brane, and comes into contact with the cell contents. As a matter 
of fact, carbon, dioxide gas diffuses into the chloroplasts. 

Problem 4. How do raw materials move in the plant? 

We have discussed the movement of the raw materials used by 
the green plant in the manufacture of food into the plant from the 
outside world how water and mineral salts enter the roots, and 
how carbon dioxide enters the leaves. These raw materials are 
made into food chiefly in cells that are green, that is, contain chlo- 
rophyll. The leaves are the chief food-making organs of the plant. 
Consequently, it will be necessary that we explain how water and 
mineral salts, once they are in the root hairs, move from the root 
hairs to the leaves. In order to explain this movement, we should 
know something of the structure of stems as well as of roots, for it 
is in them that the materials move to the leaves. 

Types of stems. We are familiar with the different kinds of 
common stems. For example, there are stems of the woody type, 
represented by the trees and shrubs, and stems of the herbaceous 
type, represented by the many thousands of different kinds of 


annual plants. Herbs have comparatively little hard, woody 

Exercise 39. Types of stems. Make a list of ten plants of woody type 
and another of ten of the herbaceous type. 

As to form, we recognize three common types of stems, namely, 
erect, prostrate, and climbing. The majority of plants have erect 
steins. The student can readily name many among herbs as well 
as among trees. Such plants as cucumber, squash, field morning- 
glory, and strawberry have prostrate stems. Examples of plants 
which climb upon other plants and depend upon them for mechan- 
ical support are grape, woodbine, hop, and scarlet runner bean. 

It is obvious that one of the chief functions of all stems, in 
addition to that of conduction of materials, is to support the leaves, 
raising them into the light and air. But many stems are also 
food-storage organs. For example, in the stems of all trees and 
shrubs, large amounts of food are stored. Also, many plants have 
sterns underground, as well as above ground, and such stems are 
usually storehouses for food. Examples of such are the tubers 
of Irish potato, the bulbs of onion and Narcissus, the corm of 
gladiolus, and the rhizome of Solomon's seal, of Canada thistle, 
of field morning-glory, and many other plants. Of these we will 
speak in another unit. 

Structure of stems. Simple water plants have little need for 
conducting tissue, for the plants are surrounded on all sides by 
water, containing in solution the various mineral salts necessary 
for plant life. Likewise those plants which grow close to moist 
soil, such as mosses and liverworts, do not require an extensive con- 
ducting tissue. But in land plants, with their leaves raised into 
the air, and with a great amount of tissue far distant from the 
source of water and mineral salts, there is need for an efficient 
means of conducting these materials throughout the plant. 

In our study of the structure of stems, let us take three differ- 
ent cases: (1) the herbaceous stem of sunflower, (2) the woody 
stem of box elder, or cottonwood, or other common broad-leaved 
tree, (3) the stem of corn. 

Exercise 40. The structure of the herbaceous stem of sunflower. Examine 
prepared cross-sections and lengthwise sections. Observe the following parts: 


(1) epidermis, a single layer of cells which protect the underlying tissues 
from drying out and from mechanical injury; (2) the cortex, composed chiefly 
of parenchyma tissue, also fiber groups, and just beneath the epidermis, a 
tissue with thickenings in the cell corners, known as collenchyma; (3) the 
vascular bundles, each with an outer region, the phloem, an inner more 
woody region, the xylem, and between them a growing tissue, the cambium; 
(4) central pith; and (5) broad vascular rays extending between the vascular 
bundles. Make a large diagram of the cross-section showing the relationship 
of stem regions, without showing cellular detail. Label. Compare the 
appearance of the different tissues as seen in cross- and lengthwise sections. 

Exercise 41. Structure of a woody stem. Study cross- and lengthwise 
sections of a one-year-old woody stem (Fig. 31). Note three distinct regions 
the bark, the wood, and the pith. The bark may be peeled away from the 
wood. It separates from the wood along a region known as the cambiunic 
The outer part of the bark is a layer of cork, beneath which are several layers of 
cells containing chlorophyll. The inner part of the bark is the phloem. In the 
woody part of the stem note the vascular rays, radiating from the pith. Make a 
diagram of the cross-section, labeling all parts. 

Exercise 42. Structure of the corn stem. Study prepared cross-sections. 
Observe the epidermis, the cortex, and a large number of vascular bundle? 
scattered in parenchyma tissue. Make a diagram, labeling all parts. 

Before studying in greater detail the structure of the stem, let us perform 
an experiment which will demonstrate in which tissues of the stem water moves. 

Exercise 43. To find out through what region water rises in stems. Place 
a fresh, leafy twig with the cut end in a solution of eosin. After several hours, 
cut sections at intervals along the stem and observe the course the liquid has 
taken. Repeat the experiment, using some herbaceous stem. Are the veins 
stained red? In another experiment remove a ring of bark from a leafy twig, 
and place the cut end, below the girdle, in water. Do the leaves wilt? From 
these experiments, what is your conclusion as to the tissues in which water is 

Exercise 44. Structure of vascular bundles. Study once more the stem 
slides of Exercises 40, 41, and 42, paying particular attention to the vascular 
bundles. Observe that the bundles of sunflower and the woody stem have a 
cambium between the phloem and xylcm, whereas there is no cambium in the 
vascular bundles of corn. The principal structural elements of the xylem are 
the vessels in which water and mineral salts are conducted, wood fibers which 
give strength and support, and wood parenchyma, a tissue which stores food. 
The principal structural elements of the phloem are the sieve tubes in which 
foods move, companion cells adjoining the sieve tubes, phloem fibers, and 
phloem parenchyma. Make a drawing of a single vascular bundle of corn or 
sunflower as seen under the high power of the compound microscope. Label. 

You will remember from your study of root structure that there 
is a central cylinder in which vascular tissue is found, a cortex, sur- 

Sec-Han of Second Year Ste 


FIG. 31. Growth in diameter of a stem of box elder. Sections made June 25, 
1934, at different points of the same branch. After studying the diagram 
in connection with the description in the context of how growth in diameter 
of stems takes place, you should be able to answer the following questions: 
At what place in the stem are new elements of wood and new elements of 
bark added? How can a cambium cell form a xylem vessel in the wood? 
How can a cambium cell form a phloem (sieve) tube in the bark? What 
makes it possible to tell from the section where growth ended in the wood 
at the end of the growing season of 1933? How can you tell where growth 
in diameter began in 1934? Why is the 1933 layer of wood thicker than 
the 1934 layer? Why is the bark in a two-year-old stem thicker than that 
of a one-year-old stem? 


rounding the central cylinder, and an epidermis, including root hairs. 
These structures of the root connect corresponding structures in 
the stem. After water has moved into the root hairs, it proceeds by 
diffusion from root-hair cells to adjacent cortex cells, and by the 
same process from one cortex cell to another until it reaches the 
vessels in the xylem of the vascular region. In the vessels it 
moves rapidly upward through the root into the stem, and thence 
on upward in the vessels of the stem to the leaves. The veins 
of the leaves are vascular bundles, and they are directly con- 
tinuous with the vascular bundles of the stem. So that water 
and mineral salts move out through the petiole of the leaf into 
all the veins. From the veins, these raw materials diffuse outward 
into mesophyll cells. 

Exercise 45. Leaf veins. (Fig. 30.) Treat small thin leaves with warm 
8d per cent alcohol until the chlorophyll is gone. Place in a shallow dish 
of water, and examine with high magnification of binocular dissecting micro- 
scope. Observe the network of veins, and the vein endings. Picture to 
yourself the movement of raw materials outward in these small veins, and 
their diffusion into adjoining mesophyll cells. 

The process of movement of raw materials in the plant. In 

the last paragraph we have given the path of movement of water 
and mineral salts from the soil to the mesophyll cells of the leaf. 
This path briefly is as follows; soil root hair cortex vessels 
of root vessels of stem vessels of leaf petiole vessels and 
tracheids of leaf veins mesophyll cells of leaf. 

What are the forces concerned in the movement of a body of 
water upward in the plant? When we consider the height of our 
tallest trees, we realize that it must require an enormous force to 
bring water to their leaves. We should say at the start that there 
is still much controversy as to the cause of the rise of sap in plants. 
We will give briefly what seems to be the most plausible and widely 
accepted explanation. 

Exercise 46. The pull of water in stems due to loss of water from the 
leaves. Arrange a demonstration of the lifting power of transpiring leaves. 
Secure a shoot with a long stem, and cut it off to a proper length under water. 
Fill a tube with water, and insert the twig, previously pushed through a 
rubber stopper. Make sure that the stopper fits very tightly into the tube, 
and that the twig fits tightly into the stopper. Sealing wax may be used 
to insure a water-tight connection. Dip the lower end of the tube into a 


dish of mercury. As the shoot loses water, it is withdrawn from the water 
of the tube, and mercury is drawn upward. Observe and record results. 

From this experiment, it will be seen that when the leaves of a 
plant lose water to the atmosphere, they exert a pull on the water 
in the conducting system of the plant. It appears that the water 
column in the plant is continuous and unbroken from the leaves 
to the roots. The pull exerted in the leaves is transferred all along 
the conducting system to the roots. If a leaf cell loses water to 
the atmosphere, the sap in that cell becomes more concentrated: 
by virtue of the greater concentration of the sap, water passes from 
adjoining cells, the sap of which in turn becomes more concen- 
trated, and so on to the conducting vessels in the leaves. Hence 
there is exerted, at the top of the continuous water column, a pull 
which is transmitted downward throughout the whole plant. It 
used to be thought that the water in a stem was " pushed " 
upward by what is called a " root pressure. " Although root pres- 
sure may play a small part in the rise of sap, there is more sub- 
stantial proof that the rise is due to a pulling force. The initial 
pulling force is created by the loss of water from leaf cells. 

Problem 5. What are the processes of food building? 

In the preceding paragraphs we have discussed the movement 
of the water and mineral salts from the soil to the mesophyll cells 
of the leaf, and also the movement of carbon dioxide from the 
atmosphere to these same mesophyll cells. Here, the raw mate- 
rials used in food-making meet, and here the process of food man- 
ufacture goes on. 

The role of light in food-building. In any manufacturing 
process, energy is required. In the primary food-making process, 
light is the energy used ; it is absorbed by the green coloring mate- 
rials. The living substances make use of the energy in building a 
food from the raw materials, carbon dioxide and water. The 
manufactured product is a simple sugar. The elements carbon 
and oxygen, of carbon dioxide, have been united to the elements 
hydrogen and oxygen, of water, in such a way as to form sugar. 
Some oxygen is left over from this process, and most of this 
passes out into the air. 


Exercise 47. To find out whether leaves make starch only when exposed 
to light. It can easily be determined whether light is necessary for the manu- 
facture of carbohydrates in leaves. We have learned that sugar is probably 
the first carbohydrate manufactured ; as a rule, some of it is changed to starch 
and accumulates during the day in the leaf. The presence of starch may be 
detected by treating the leaf with iodine, which turns starch grains bluish- 
purple. A simple experiment consists in covering a portion of a leaf attached 
to growing plant, which has been in the dark a day, with tinfoil to exclude the 
light. Then expose the plant to the light for several hours. At the end of 
that period, remove the leaf from the plant, extract the chlorophyll by warming 
the leaf in alcohol, and then treat the leaf with iodine. What is the action of 
the iodine on the portion of the leaf which was uncovered? What does this 
indicate? What is the effect of the iodine on the part which was covered? 

Exercise 48. The rdle of chlorophyll in food-building. To find out whether 
it is only in the green parts of leaves that starch is made, extract the clorophyll 
from a variegated leaf of Coleus, or other plant which has white areas in it, 
which has been exposed to light for several hours, and treat with iodine. Note 
the effect of the starch on the parts of the leaf which were white and on the 
portions of the leaf which were green. Write a paragraph explaining the results 
of this experiment. 

The end-products and by-products. In most plants the first 
visible product of photosynthesis is starch. In many of the 
Musaceae (banana family), starch is absent from the leaves, oil 
being the first visible product of photosynthesis. In many other 
monocotyledons there is no visible product of photosynthesis to 
be found within the green cells, for the sugar produced remains in 

The sugar which is the immediate product of photosynthesis 
is generally assumed to be glucose (C(;Hi 2 6 ). There is some 
evidence, however, that it maybe sucrose (cane sugar) (Ci2H 22 Ou). 
Both of these sugars are present in considerable quantities in 

The equation which has been used to express in simple form 
the chemical changes which take place during photosynthesis 
(6CO 2 + 6H 2 O - C 6 Hi 2 O 6 + 6O 2 ) shows that oxygen is freed 
during the process. This gas is a by-product of photosynthesis. 

Exercise 49. To determine whether oxygen is given off in photosynthesis. 

Tn a gallon battery jar place a bunch of fresh, active Elodea plants. Keep in 
sunlight. Over them invert a wide funnel, and over the tube of the funnel 


place a test tube filled with water, so arranged as to catch the gas bubbles which 
are given off. Apply the oxygen test to the gas. 

The process summarized. We see that, in the process of car- 
bohydrate manufacture or photosynthesis carried on only by 
green plants, and only in those tissues of green plants which possess 
chlorophyll, carbon dioxide and water are the raw materials; 
sunlight, the energy; special living green bodies chiefly in leaf 
cells, the factory or laboratory; sugar, the final product; and 
oxygen, the by-product. Carbohydrate manufacture is a process 
carried on only in cells containing a green coloring matter and 
only in the presence of light. 

Careful measurements have been made of the amount of light 
energy which is absorbed by the green leaf, and of the quantity of 
this actually used in food-making. It has been found that about 
10 per cent of the total light energy which falls upon the leaf is 
absorbed by the green coloring matter, and that of this amount 
only about 35 per cent is used in food-making. This means that 
approximately 3.5 per cent of the energy falling on the leaf is 
utilized in carbohydrate building. 

Problem 6. What use does the plant make of the food manufac- 
tured in green tissue? 

The sugar, glucose, which is thought to be the immediate 
product of the photosynthetic process, is the foundation material 
used in the synthesis of most other plant substances. Part of 
this simple carbohydrate is respired, liberating energy to be used 
by the plant in doing work. Some of it is stored as such, to be 
changed later into various other substances. 

Respiration. Respiration is one of the vital processes in 
plants. In all essential particulars the process is the same in 
plants as in animals. It is true that plants do not have organs 
in any way resembling lungs, which serve to facilitate the exchange 
of gases between the atmosphere and the cells of the body, but the 
essential features of respiration are the same in both plants and 

Respiration is a process which goes on only in the living cells. 
Every cell of the plant body must respire if it is to maintain its life. 


We have often been led to believe that leaves are the respiring 
organs of the plant. It is true that respiration is quite rapid in 
leaves, but they are no more the respiring organs of the plant than 
are the stems, the roots, or other living parts. Moreover, respira- 
tion is not going on any more rapidly in leaves than in many other 
living organs of the plant. It is in all living cells, no matter in 
what organs or tissues they may be found, that respiration takes 

Respiration is a process in which substances of the plants, such 
as sugars, are broken down by the aid of oxygen into simpler 
products, the principal ones of which are carbon dioxide and water. 
It is a destructive process. In this breaking-down process, energy 
is liberated. Some of the energy of respiration is used directly by 
the living cell for the processes which are essential to its life; the 
remainder is lost as heat. In respiration, plant foods are used up, 
being oxidized or " burned " by means of oxygen. In many 
respects respiration is similar to combustion. As far as each living 
cell or the entire plant body is concerned, the exchange of gases 
involves an intake of oxygen and an outgo of carbon dioxide. 

There may be some confusion regarding two of the important 
processes of the green plant, namely, carbohydrate manufacture 
(photosynthesis) and respiration. In preceding pages the carbo- 
hydrate-manufacturing process of plants was described. This 
process, too, goes on in the living cells, but only in those cells 
which are exposed to light and contain a green coloring matter 
(chlorophyll). And, in the carbohydrate-manufacturing process, 
carbon dioxide is taken in, and oxygen given off. This is just the 
reverse of the gas exchange in respiration. Carbohydrate manu- 
facture takes place in a relatively few cells of the plant, and only 
during the day. Respiration, on the other hand, proceeds day 
and night in all living cells, whether they contain chlorophyll or 
not. Again, we should recognize that whereas respiration is a 
food-destroying or energy-releasing process, that process peculiar 
to green cells is a food-building or energy-storing process. 

It is very probable that there are times during the day wheil 
respiration and carbohydrate manufacture go on at about the 
same rate in the green tissues of the plant. At such times, the 
oxygen set free in the latter process is not lost to the atmosphere but 


is immediately utilized by the cells in respiration, and the carbon 
dioxide eliminated in respiration is taken by the green cells and 
used in the process of carbohydrate manufacture. But during 
the night, when the utilization of carbon dioxide in food-building 
ceases, this gas escapes from the plant to the atmosphere. In an 
actively growing green plant, the amount of oxygen liberated to 
the atmosphere by the carbohydrate-manufacturing process during 
the twenty-four hours exceeds that absorbed in respiration, and 
the carbon dioxide contributed to the atmosphere by the respira- 
tion of such a plant is much less than that absorbed during carbo- 
hydrate manufacture. Thus, we see that green plants play a 
great part in the scheme of nature, in that they maintain a proper 
ratio of the important gases of the atmosphere, by removing carbon 
dioxide from it and adding oxygen. It has been computed that 
approximately 280 square feet of green leaf surface will give out, 
during a moderately warm and sunny day, the quantity of oxygen 
used by a man for respiration during the same period. 

Materials which compose the plant skeleton. Glucose may 
be changed into cellulose, a carbohydrate which enters into the 
structure of cell walls. Cellulose is the most abundant carbo- 
hydrate in the plant kingdom. Cellulose is the basis of a large 
number of commercial products such as paper, explosives, cello- 
phane, celluloid, rayon, etc. Glucose may be changed into pectic 
substances. Pectic compounds occur in the cell walls of many 
fruits, breaking down in boiling to form a jelly. 

Reserve foods. Glucose may be changed into other sugars 
such as fructose and sucrose. Fructose is common in the fruits 
of plants. Sucrose is a reserve food, particularly abundant in the 
root of sugar beet and in the stems of sugar cane. 

Glucose, or other carbohydrates, may be converted into fats 
or oils. Fats and oils are especially abundant in seeds and fruits. 
Common oils of commerce are castor, linseed, cottonseed, olive, 
coconut, and peanut. State the commercial uses of these different 
kinds of plant oils. 

The proteins, substances of importance in such seed as beans, 
peas, and cereals, are also in large part built from carbohydrates, 
chiefly glucose. In addition to the elements carbon, hydrogen, 


and oxygen, which occur in all carbohydrates, proteins also contain 
nitrogen, sulphur, and some of them phosphorus. These three 
elements are derived from mineral salts which come from the soil. 
Frequently we have mentioned the mineral salts as raw materials 
in the food-manufacturing process. They do not form any part 
of glucose, but they do enter into the make-up of all plant proteins 
and many other important plant substances. 

Various secretions and other substances. But these foods 
carbohydrates, fats, and proteins are not the only plant sub- 
stances of economic importance. Consider the various plant pig- 
ments, the resins and gums, the milky latex of many plants, some 
of which yield the latex from which rubber is made, and the 
alkaloids nitrogenous substances such as quinine (from bark of 
cinchona), caffeine (from coffee), thein (from tea), morphine 
(from the poppy), nicotine (from tobacco), and atropin (from 
nightshade). Also a large number of organic acids are found in 
plants, such as citric acid (from citrus fruits) ; there are innumer- 
able essential plant oils, such as lemon oil, cedar oil, clove oil, va- 
nilla, camphor, etc. Also, consider the great commercial impor- 
tance of tannin, a bitter substance found in the bark of many 
trees and employed in the tanning of leather. 

Thus we see that the sugar, glucose, manufactured only in 
green cells, forms the foundation of many other plant foods, and 
other plant substances that probably cannot be classed as foods. 
Glucose is manufactured in the cells only when they are exposed 
to light. However, all the other chemical changes in the plant, 
including the synthesis of fats, of proteins, of alkaloids, of acids, or 
essential oils, and other substances, are independent of direct sun- 
light. Sunlight is directly necessary for only one process, namely, 
photosynthesis, or the building of glucose out of carbon dioxide 
and water. We have seen that glucose manufacture utilizes the 
energy absorbed from light. In other words, in glucose manufac- 
ture radiant energy is transformed into the chemical or potential 
energy of the glucose molecule. In all other chemical changes in 
the plant, for example, the synthesis of proteins, the energy for 
doing the work is derived from respiration. 


Problem 7. Whaf is the role of the different elements in the 
nutrition of green plants? 

In the preceding sections it was emphasized that only green 
plants have the power of manufacturing, from the simple com- 
pounds derived from the soil and atmosphere, the foods which 
are used in nourishing the plant body. 

We have also seen that this food-manufacturing process is of 
great significance, in that the foods constructed furnish the mate- 
rial out of which the bodies of both plants and animals are built; 
and moreover, in the making of these foods by green plants, 
energy from the sun, which is the world's great and only source of 
energy, is stored. 

It has long been realized by agriculturists that a proper fer- 
tilizer practice was dependent upon a knowledge of the influence 
which the different chemical elements exert upon the plant's 
growth. If, for example, we would apply nitrates to the soil, we 
should know how this salt is going to affect the crop. Moreover, 
we should be able to tell when a crop is suffering from a deficiency 
of any essential chemical element, and what the effects are of 
an excess. 

Principal substances used by green plants. It was also 
pointed out that there were certain substances which the plant 
must have in order to maintain life. The principal substances 
which are taken into the green plant, and in some way made use 
of, are as follows: 

From the soil. (1) Water, and (2) salts containing principally 
nitrogen, phosphorus, sulphur, potassium, calcium, magnesium, 
and iron. From the atmosphere. (1) Carbon dioxide and (2) 

Let us briefly discuss the part that each of these substances 
plays in the life of the plant. 

Water. The living material (protoplasm) of the plant is 80 
to 90 per cent water. We have learned that water is an essential 
raw material for the manufacture of sugar. Water is the solvent 
of the gases, oxygen and carbon dioxide, and also of all mineral 
salts. None of these substances can enter the plant unless they 
are in solution. We have seen that raw materials and foods move 


from one part of the plant to another in watery solution. The 
cells of the plant function normally only when distended with 

Nitrogen. Nitrogen is a constituent of all proteins, which are 
essential components of protoplasm. Protein contains 15 to 19 
per cent of nitrogen. It is well known that an abundance of nitro- 
gen tends to produce a rank growth of leaves, stems, and roots 
and to retard the date of maturing of the plant. Crops grown for 
their leaves only are improved by applications of nitrate. How- 
ever, an excess of nitrogen in such a crop as cabbage may result 
in a softness and tenderness which make it difficult to ship and 
keep well. Cereal crops produce, as a rule, too much straw, and 
" lodge " badly if there is an excessive supply of nitrogen. An 
excess of nitrogen applied to potatoes stimulates a leafy growth, 
but not a proportionate weight of tuber; applied to tomatoes, it 
produces too much leaf and too little fruit; applied to sugar beets, 
it results in high tonnage, but reduced sugar content. Heavy ap- 
plications of nitrogenous fertilizers to fruit-bearing plants may 
cause increased vegetative growth, which is usually associated 
with decreased fruit production. Nitrates are by far the most 
available source of nitrogen for crop plants. 

Phosphorus. Like nitrogen, phosphorus is a constituent of 
proteins. It is essential to a rapid multiplication of cells. It is 
known that such insoluble carbohydrates as starch are not changed 
into the soluble form (sugar) unless phosphorus is present. In 
the early stages of the plant's growth, phosphorus promotes devel- 
opment. The fact that applications of phosphorus to the soil 
hasten the maturity of plants is probably due to the stimulation 
of rapid early growth. On heavy soils, where roots do not form 
well, phosphorus stimulates root development. Plants secure 
their phosphorus from the soil phosphates. 

Sulphur. This is an indispensable element in plant growth. 
It is essential to the formation of proteins. A deficiency of sulphur 
results in a failure of the cells to divide at a normal rate, and in a 
suppression of fruit development. The characteristic flavor of 
onions and garlic is due to certain sulphur compounds. Plants 
secure sulphur from the sulphates of potassium, calcium, magne- 
sium, and iron. 


Potassium. Potassium is essential to the manufacture and 
movement of carbohydrates. Such plants as sugar beets, pota- 
toes, and others which manufacture and store large quantities of 
carbohydrates are particularly responsive to the available supply of 
potassium. This element has a marked effect on the weight of 
grain. Potash starvation shows in the dull, yellowish color of the 
foliage, in a loss of vigor, and a lessened resistance to disease. 

Calcium. This element seems to stimulate root growth. A 
deficiency retards the movement of carbohydrates in the plant 
and their utilization by the plant. Calcium aids in neutralizing 
acids, both without and within the plant, which might limit the 
growth. Calcium enters into the composition of the middle mem- 
brane of cell walls. 

Magnesium. It is now known that magnesium is necessary 
for the formation of the green coloring matter (chlorophyll) of 
plants. In fact, it is a component of the green coloring matter. 
A deficiency of magnesium results in pale, colorless foliage. Mag- 
nesium also appears to aid in the movement of phosphorus in 
the plant. 

Iron. Although iron does not enter into the composition of 
chlorophyll, it is absolutely essential to its formation. Even in 
the light, plants become pale when grown without iron. Very 
small amounts of iron salts in the soil are sufficient. 

Carbon. As has been stated, green plants derive all their 
carbon from the air in the form of carbon dioxide. Carbon enters 
into the composition of all carbohydrates, such as sugars, starches, 
cell walls, and is also an essential component of fats, of proteins, 
and of living material itself. Carbon makes up from 40 to 50 
per cent of the dry weight of all plants. 

Oxygen. This element enters into many chemical compounds, 
but in its elemental form is essential in the process of respiration. 
This important process will be discussed later. 

It should be understood that the plant is taking in a great 
many more chemical elements than those mentioned in the preced- 
ing paragraphs. The fact is that an analysis of plant ash reveals 
the presence of most of the elements which occur in the soil. How- 
ever, it is not known what part, if any, many of the rarer elements 
play in the plant's life. It may be that some of them, like iron, 


even in small traces, are indispensable to normal plant growth, or 
at least influence the plant's development. Experiments seem 
to bear out the truth of this statement. It should also be pointed 
out that the salts of the soil are ordinarily in very dilute solutions, 
and are taken in by the plant in small quantities. 

Problem 8. Where do foods move in the plant? 

Although all living cells of the plant contain sap, not all of them 
are concerned in its rapid movement throughout the plant. In all 
stems and roots, there is an upward-moving sap stream and a 
downward-moving sap stream, and these differ in their chemical 
composition. The upward-moving stream is mainly water and 
mineral salts from the soil, and food substances which have been 
stored in roots and stems; the downward-moving stream carries 
food substances, dissolved, of course, in water. 

We learned that the conductive tissues of the plant are grouped 
into bundles called vascular bundles. Each bundle is composed 
of three groups of structural elements, the xylem, the phloem, and, 
in most plants, a cambium between the xylem and phloem. We 
learned that water and mineral salts moved in the vessels and 
tracheids of the xylem. That is, the upward-moving sap stream 
is in the xylem or woody portion of the stem. Now recall that 
the conducting elements in the phloem are sieve tubes. Gird- 
ling experiments with stems show that foods, chiefly sugars, are 
transported in the sieve tubes of the phloem. That is, the down- 
ward-moving sap stream is in the phloem or bark portion of the stern. 
And bear in mind that all foods which are moving from one part 
of the plant to another are in solution. Starch grains or protein 
granules cannot move as such throughout the plant. Why? 

In girdling, the bark is cut completely around the stem down 
to the wood. That this operation does not stop the upward flow 
of water is evidenced by the fact that the leaves do not wilt. But 
foods do not move past the girdle. This indicates that their con- 
duction is in the bark. If the main trunk of a tree is girdled, the 
roots are starved for want of food, and the tree finally dies. The 
girdling of stems often results in increased growth above the girdle. 
This condition also seems to show that there is a downward move- 


ment of foods in the bark, and that they have a tendency to 
accumulate above the girdle, thus supplying material for addi- 
tional growth. 

The path of movement of the food manufactured in the meso- 
phyll cells of the leaf is probably as follows: It diffuses into cells 
joining the vein endings in the leaves, from one cell to another, 
until it comes to sieve tubes; once in the sieve tubes it is free to 
move rather rapidly to all parts of the plant, passing down into the 
petiole, thence to the stem and roots. 

Problem 9. How does the plant store and digest its food? 

As a rule, the food that is manufactured by a plant accumulates 
faster than it is needed. Accordingly, there is some provision for 
the storage of food. The amount of food stored and the place 
of storage depend somewhat upon the length of life of the plant. 
For example, in annual plants, those that live but one year, the 
supply of stored food is confined to the seeds. In biennials, plants 
that live two years, producing seed at the end of the second year, 
not only is food stored in seeds, but also large amounts occur in 
roots. Examples of such plants are carrot, parsnip, turnip, sugar 
beet, etc. In perennials, plants that live from year to year, food 
is stored not only in seeds, but also in large quantities in roots and 
stems. For example, the dandelion root is perennial, and at all 
times has a supply of food in reserve. Well do we know this, for 
if the dandelion is cut off, new shoots promptly arise, making their 
growth at the expense of food stored in the roots. Many perennial 
weeds have underground stems; examples are wild morning-glory 
or bindweed, Canada thistle, and Russian knapweed; and many 
common economic plants, such as Solomon's seal, Trillium, certain 
larkspurs, Irish potato, Jerusalem artichoke, asparagus, and 
others also have underground stems. In all these plants, the 
underground stems are food-storage organs. In woody plants, the 
trees and shrubs, food is stored in twigs and branches of all sizes, 
in the main trunk, and in the roots. In fact, a tree lays up for the 
dormant season an enormous reserve of food which, in the spring, 
moves into all the buds, furnishing nourishment for their early 
growth. In the woody stems and roots, foods may be stored in 


the vascular x rays, in wood parenchyma, in phloem parenchyma, in 
cortex, and in pith. 

The kinds of stored foods. The principal stored foods are 
starch, sugars, proteins, and oils. Probably the most common 
food stored in plants is starch, as for example in the seeds of corn, 
wheat, oats, and rice, the tuber of potato, and in the roots and 
stems of woody plants. Starch occurs in the form of grains, the 
shape, markings, and structure of which are characteristic of each 

Exercise 60. Starch test. Apply the starch test to sections of a number of 
different kinds of structures including seeds, roots, and stems. Write up your 

When photosynthesis is actively going on, starch usually accu- 
mulates in the leaf, and can be detected by applying the iodine test 
described on page 40. This accumulation means that glucose is 
being made more rapidly than it can be carried aw T ay, and that it 
is changed to starch and stored as such temporarily. That this is 
so is borne out by the following simple experiment. Small portions 
of a leaf tested for starch in the evening after a day of photo- 
synthetic activity show starch. If pieces of the leaf are taken in 
the morning before it is light, or if the leaf is covered with opaque 
paper in the evening, so that light does not strike it in the morning 
before the sample for testing is made, a test for starch is negative. 
Evidently during the night stored food has moved out of the leaf 
to various parts of the plant. Thus starch may be a temporary 
storage product of leaves. 

Sugars, chiefly glucose, fructose, and sucrose, are very common 
stored foods. Usually they may be detected in the sap of cells 
in almost any part of the plant. They are stored in large quanti- 
ties in certain fruits and in some vegetative structures. Notable 
examples are the roots of sugar beet and the stems of sugar cane, 
which may contain from 15 to 20 per cent of sucrose. 

Proteins are also an extremely common storage product, espe- 
cially in seeds, particularly such seeds as beans and peas. Oil is a 
reserve food in such seeds as flax, cotton, olive, and peanut. 

The process of food digestion. Plants digest their foods, and 
essentially in much the same manner as animals. As an example, 


let us consider starch. We have learned that substances which 
move from place to place in the plant must be in solution. Starch 
grains can not move through cell walls and protoplasmic mem- 
branes. Consequently, starch stored at points in the plant far 
removed from growing points, where it is most needed, must first 
be changed into some form which will diffuse through cell walls and 
protoplasmic membranes. This change of starch, a substance 
which is not soluble in the cell sap, to a material which is soluble 
in the cell sap, is a process called digestion. 

Digestion is brought about by various complex substances 
known as enzymes. They are protein in character. They are 
usually present in very small amounts in cells, but it ap- 
pears that even a small quantity may be sufficient to bring 
about the digestion of a relatively large amount of material. 
Moreover, there is no appreciable decrease in the amount of 
enzyme as a result of its action. There is a specific enzyme, 
known as diastase, through the action of which starch is changed 
to sugar, glucose. Diastase has no other digestive function; its 
action is specific. 

Let us illustrate the processes of digestion, movement of food, 
and storage of food as they occur in the potato plant. The cells 
of the potato plant which contain chlorophyll manufacture sugar 
from carbon dioxide and water. Some of this sugar, as such, is 
immediately conveyed from the leaf and goes to various parts 
of the plant where it is used in respiration and to nourish the 
tissues. Some of it reaches the developing tubers underground. 
Some of it may form the basis for fats and proteins. And a large 
proportion of it is converted to starch and temporarily stored in 
leaf cells. At night, when the sugar-making process has stopped 
on account of the lack of light, the temporarily stored starch is 
digested, that is, converted to a soluble form, which in this case 
is sugar. The agency causing this change is the enzyme diastase. 
The sugar moves out of the leaf, through the leaf stalk into the 
stem, down through conducting tubes of the vascular bundles, and 
into the tubers. This sugar is converted back into starch and 
stored as such in the tuber. The potato tuber is a starch-storing 
organ. When the tuber is planted and begins to sprout, it becomes 
sweet, indicating that starch is being converted to sugar. Further- 


more, actual test shows sugar on the increase and in transport to 
the developing sprouts. 

As another illustration, let us start with starch stored in the 
vascular rays of a peach twig. Vascular ray cells are living; in 
fact, food storage occurs only in living cells. When temperature 
and other conditions are favorable in the spring, these vascular ray 
cells begin the secretion of diastase, and stored starch is changed 
to sugar, that is, starch is digested. The sugar diffuses from vas- 
cular ray cells into adjoining tissues and finally reaches the growing 
cells in the buds. 

There are many different kinds of enzymes, each having a 
rather specific function. Lipase is an enzyme which facilitates the 
breaking down of fats into glycerin and fatty acids; pepsin digests 
proteins, converting them into water-soluble peptones and pro- 
teoses; trypsin digests peptones and proteoscs, changing them 
into amino acids. 

Enzymes may be secreted by any living cells of the plant, or 
wherever digestion is necessary. 

Exercise 61. Starch digestion. Place some starch in a small, shallow 
dish and cover with a solution of diastase, which may be obtained as a com- 
mercial product. Keep at a temperature of about 75 to 80 F. for 12 to 24 
hours. Examine the starch grains with the high power of a compound micro- 
scope, and observe that they are " eaten " and corroded. If facilities permit, 
test some of the solution for sugar. 

Problem 10. How does the plant assimilate food? 

Up to this point in our discussion of the nutrition of green 
plants we have discussed the processes of food manufacture, its 
storage, its digestion, its use, and its movement within the plant. 
Digested food within the plant cell is not a part of the living 
protoplasm. One more step is necessary that of making it a 
part of the living protoplasm itself. It must be changed from 
lifeless food to living protoplasm. This process is called assimila- 
tion. The nature of this transformation is not understood. But 
we may be assured that everywhere throughout the plant, in 
living cells, there is going on this marvelous change of non-living 
foods to living stuff. But the change is brought about only by 
the action of other living matter already existing. 



1. Define organic substances and inorganic substances, and cite examples 
of each. 

2. Which classes of substances nourish the bodies of plants and animals? 

3. What is the great role that green plants play in the world's economy? 

4. What inorganic compounds do living plants give off? 

5. What elements do carbohydrates contain? 

6. Why is a sprouted potato sweeter than an unsprouted one? 

7. Sweet-corn kernels contain much more sugar than field-corn kernels. 
Do you see any relation between this fact and the wrinkledness of dry sweet 

8. What is meant by an independent plant? 

9. Define assimilation. 

10. Discuss digestion in relation to seed germination. 

11. When slices of red beet are placed in water what prevents the coloring 
matter from diffusing out? Why does the coloring material come out when the 
beet is boiled? 

12. What is the difference between the food of plants and that of animals? 

13. What gases enter a green leaf in sunlight? 

14. What is an enzyme? 

15. Explain why an apple tree dies eventually when a ring of bark is removed 
from the main stem. 

16. Explain why a tree girdled in summer may live and remain green during 
the remainder of the season but fail to leaf out the following spring. 


The Green Leaf, by D. T. MACDOUGAL, published by D. Appleton and 
Company, 1930, 142 pages, 22 figures. Mention of chapter headings shows 
how much there is of interest in this little volume : living matter from rocks, 
water and air; place in the sun; models of sun-screens and our utilization of 
their products; the grass blade; pine needles; tree records of climate; the 
oak leaf; movements of sap and autumnal colors; green stems; a visit to 
green leaf mills; green mills and their grist; protoplasm, how it started and 
how it goes; growth; ghost and other dwellers in darkness; leaf-products 
and human populations. 


We learned in Unit II that green plants make their own food. 
For this reason they are called independent plants, or autophytes. 
Plants without chlorophyll and the animal life of the earth are 
dependent, since they have not the power to make food, but must 
get their food from supplies furnished, directly or indirectly, by 
green plants. 

Almost all the non-green plants are included in the groups 
which we know as bacteria, yeasts, molds, mildews, rusts, smuts, 
and mushrooms. Certain of these plants are related in an impor- 
tant way to man's welfare. They get into his foods which are 
unprotected and cause them to spoil. They enter his body and 
the bodies of animals and produce poisonous substances that 
cause disease. In a similar way they affect plants, causing serious 
damage to cereals, fruits, and other crops. Through unceasing 
effort scientists have learned much about the nature of these plants 
and about how they live, and this knowledge has been a help to 
mankind in protecting against the injurious forms. 

Although certain of the bacteria, yeasts, and molds are man's 
enemies, we should know, also, that representatives of these plant 
groups are absolutely essential to the existence of other life on 
the earth. Without the help of the microscopic forms of life 
which are working quietly within the soil, the continued existence 
of man and his civilization would be impossible. Certain of the 
soil bacteria and molds cause the decay of dead plant and animal 
bodies. Others use the nitrogen of the air in making new chemical 
compounds which are essential to the growth of other plants. In 
this way the soil is kept fertile, making possible the production of 
plants year after year. 

In general, the non-green plants are simple forms without 
roots, stems, or leaves; yet there are a few flowering plants which 



lack chlorophyll and so must depend upon green plants. Among 
the dependent seed plants are the Indian pipe and pinesap, which 
derive food from dead plant materials in the soil, and beech drops 
and squaw root which live as parasites on the roots of certain 

It will be interesting to study the relationships between the 
independent life of the earth and the dependent life. Green 
plants, in the process x of food-making, use carbon dioxide in large 
amounts and give off to the atmosphere an equal volume of oxygen; 
in respiration, they use oxygen and give off carbon dioxide in small 
quantities. The ultimate effect of green plants on the air is to 
decrease the amount of carbon dioxide and increase the amount of 
oxygen. Animals and non-green plants affect the atmosphere by 
increasing the amount of carbon dioxide and decreasing the amount 
of oxygen through the process of respiration. Thus there is a 

balance in nature between 
green plants and living things 
without chlorophyll. The 
plants with chlorophyll fur- 
nish oxygen which all living 
things require and use carbon 
dioxide which is a waste prod- 
uct of all life. 

F7a. 32.-A balanced aquarium. A balanced aquarium (Fig. 

Denoyer Geppert Co. 32) is set up by adding animals 

and plants to the water in pro- 
portion so that the plants will furnish the animals the required 
amounts of food and oxygen and the animals will furnish supplies 
of carbon dioxide and other raw materials needed by the plants. 
The balanced aquarium is a miniature world with a definite 
balance between the plant life and the animal life which it 

We can consider green plants independent only in the sense 
that they are able to build foods from raw materials; they are 
dependent, in a way, since a large part of the raw materials neces- 
sary for the process of food-making is furnished by other forms of 


Problem 1. What are the main characteristics of the 
non-green plants? 

The outstanding characteristic of the non-green plants is the 
absence of chlorophyll in their tissues. In a field planted to corn 
an occasional plant appears with white leaves. No one has seen 
a full-grown stalk of corn having only white leaves. It is signifi- 
cant that these young albino corn plants live only as long as the 
food stored in the seed lasts. The parent plant had chlorophyll, 
so food could be produced and stored in the seed. The albino 
seedling, having no chlorophyll, must die as soon as it has used 
this original store. 

There are no chlorophyll-bearing forms among that simple 
group of plants known as fungi. The plant begins development 
in contact with organic material, and all its needs for producing 
the plant body and reproductive structures are supplied in the 
form of ready-made food. 

All living organisms need energy for life processes. Whether 
plant or animal, every living thing uses oxygen and gives off 
carbon dioxide. Substances are oxidized in the cells of the living 
body, and energy is released. If the living thing can not store 
energy for itself in the form of food it must get its supply of energy 
from the store provided by other living things which can make food. 

We have considered non-green plants dependent on green 
plants, but certain bacteria are exceptions. Anyone who has seen 
a mineral spring has noted the foul smell and the whitish or yel- 
lowish coating of objects in the stream of water running away from 
the spring. The same may be noted in a sluggish stream contain- 
ing sewage. The smell is due to a gas known as hydrogen sulphide 
which escapes from solution in the water. The coating of objects 
is due to sulphur bacteria. These bacteria are primitive forms 
which can live and secure energy from the oxidation of hydrogen 
sulphide to sulphur and the oxidation of sulphur to sulphuric acid. 
These or similar forms must have represented the first life on the 
earth when very few of the types of animals or plants found today 
could have survived the severe conditions. 

Saprophytes. Dependent plants which derive their food from 
non-living organic material are known as saprophytes. The 



organic matter may be either plant or animal. There are a great 
many different kinds of saprophytic fungi. Mushrooms are a 
common example. They may be grown in beds prepared from 
partially decayed stable manure mixed with rich loam. The 

decayed manure and loam 
furnish the organic material 
for the use of the mushroom. 
Enzymes are secreted by the 
part of the plant consisting of 
a network of tiny underground 
threads. These enzymes 
change the complex food sub- 
stances into simpler materials 
which can pass through the 
membranes of the plant in 
absorption. Inside the plant 
the food materials may be oxi- 
dized, or they may be assimi- 
lated, forming protoplasm 
which is used in the building 
of new plant structures. A 
saprophyte is similar to an 
animal in that it requires food 
which has been derived directly 
or indirectly from a green 

Why are mushrooms fre- 
quently found growing in the 
woods around dead trees or 
stumps? Explain why mush- 
rooms are found growing in 
soils that have humus (decay- 
ing plant material). 

During the hundreds of 
thousands of years that life 
has been in existence on the earth enormous quantities of material 
have been produced in the form of animal and plant bodies. It is 
to be noted that these materials have not accumulated on the 

FIG. 33. Shaggy-mane mushroom 
growing in a city backyard. Why is 
this mushroom growing here among 
the green plants? Does it need the 
sunlight? Are the materials which it 
takes from the soil the same as those 
taken by the green plants? Could 
plants without chlorophyll develop 
without the aid of green plants? Could 
green plants succeed without the aid 
of the non-green plants? 



surface of the earth. Without the great variety of saprophytes 
which dispose of the dead organic substances, life on the earth 
would have become impossible long ago because of the debris 
resulting from the accumulation of these materials. 

tfiG. 34. These large mushrooms are saprophytes, getting food materials 
from the humus on which they are growing. 

The annual herbaceous plants and the leaves of trees and 
shrubs fall to the ground each year, and myriads of non-green 
plants begin the process of 
transformation. The com- 
plex plant materials are at- 
tacked in the process of 
decay, and this and other 
processes which follow change 
these substances into simple 
raw materials suitable for use 
of other plants. In the same 
way, the parts of crop plants 
which are not removed from 
the fields by man are returned 
to the soil where bacteria and molds transform them into materials 
that can be used by other crops. What conditions would follow 
if all the saprophytes were suddenly destroyed? 

FIG. 35. Certain fleshy fungi live as 
saprophytes on fallen logs and on stumps. 


Saprophytes are not always beneficial to man. Good food 
for man is also suitable food for other organisms which may get in 
and spoil the food for man. It has been necessary for man to 
devise methods of food preservation such as drying, canning, use of 
chemicals, and refrigeration in order to make it impossible for 
injurious saprophytes to enter or grow in the food and thus 
destroy it. 

Exercise 52. How does a saprophyte secure food? Mount in water on a 
clean slide a small portion of rotten apple or orange, cover with cover-glass, and 
examine under the low power of the microscope. What evidence do you see 
of plant growth within the apple? What is the source of food of the structures 
which you see? How does the food material of the apple get into the plant 
body of the fungus? 

Exercise 53. How may saprophytes be spread? Examine an apple or 
orange in the advanced stages of rot for any evidence of the fungus on the 
outside of the fruit. Mount on a slide in water under a cover-glass a small 
amount of any blue or green powdery fungus material that you may find. 
What is the shape of the bodies as they appear under the microscope? What 
seems to be the relation between the structures of the plant which you find 
on the surfaces of the fruit to those which you found within? Why are the 
thread-like structures produced within the fruit rather than on its surface? 
Why were the resistant spherical structures produced on the surface of the fruit 
rather than within? Show how the part of the plant within the fruit and the 
part of the plant on the outside is in each case fitted to live where it is produced. 
How could the small spherical bodies on the outside known as spores be spread 
to other fruits? 

Exercise 54. How may saprophytes enter food materials? Place in a dish 
a sound apple in contact with the rotting portion of an affected apple. On the 
opposite side of the sound apple prick the skin with the point of a knife and 
introduce some of the spore material found on the surface of decayed fruit. 
Cover the dish and examine each day for a few days. Describe fully what 
happens to both sides of the apple which was formerly sound. What rules 
would you give for preventing decay in sound fruits, such as apples and 

Parasites. Many of the non-green plants, though dependent 
like the saprophytes, derive their food from living organic matter, 
that is, from the bodies of living plants or animals. These are 
parasites. Some of the greatest discoveries of medical science are 
concerned with methods of protecting against disease-producing 
bacteria which are parasitic plants. Many cultivated plants and 
domestic animals are attacked by plant parasites, and huge sums 


of money are expended annually by public agencies and private 
individuals in fighting these parasitic plant pests. 

The main point of difference between the saprophytes and para- 
sites is the fact that the parasites derive their food from living 
plant and animal bodies whereas the saprophytes thrive on dead 
organic materials. They are alike in the fact that both require 
organic material for their nutrition, that is, neither has the power 
to make foods from raw materials. 

Since bacteria and other fungi use the living material of plants 
and animals as food, they naturally are injurious in their relation 
to the host (the living thing in which they grow). They do injury 
by destroying the living material of the host. Also, many of them 
are injurious because they produce substances which are poisonous 
to the host. These substances are known as toxins. Both plants 
and animals are affected by toxins, but since the animal has a 
circulatory system and a plant has nothing to compare with this, 
the toxic substances can not affect the plant in the same way they 
affect animals. 

Many of the most serious diseases which attack man, such 
as typhoid fever and diphtheria, are due to the effects of bacteria. 
The non-green plants which produce disease conditions in plants 
or animals are called pathogenic forms. The science which deals 
vith plant diseases is plant pathology. 

Following the work of Louis Pasteur, a Frenchman, and Robert 
Koch, a German, showing that bacteria are the cause of many 
diseases in animals, science has built up a system of preventive 
medicine. It has been found possible in many cases to prevent 
invasion of the body by bacteria, and in others, to prevent the 
most injurious effects of those that have made a start. The 
medical doctor has come more and more to be an educator, showing 
people how to keep well. 

Suggested activites. You will find interesting accounts of the life and 
work of Robert Koch and of Louis Pasteur in Science in the Service of Health, 
by Downing (Longmans). Prepare a report to be read to the class on the 
life and work of Robert Koch. Prepare a paper on the service to mankind of 
the work of Louis Pasteur. 


Problem 2. What are the nutritive relations of the saprophytes? 

The principal saprophytic plants of economic importance are 
the bacteria, yeasts, and molds. All these groups are widely dis- 
tributed on the earth. Organic materials either of plant or animal 
origin when exposed under suitable conditions of moisture and 
temperature are soon alive with representatives of one or more 
of these forms. The material in which they grow becomes 
changed. Through the life processes of saprophytes, sweet milk 
becomes sour, alcohol is formed in fruit juices, and the alcohol 
solution may be changed to vinegar. 

Exercise 65. How are organic materials changed by saprophytes? Place 
in a fruit jar various dampened plant materials such as pieces of banana, 
apple, and bread; and other organic materials as cheese and leather. Put on 
the cover without the rubber, and leave in a dark place at living-room tem- 
perature. Examine each day and note changes in the appearance of the mate- 
rials. Describe and account for the changes from day to day. Why is apple 
or cheese a suitable material for the growth of saprophytes? Why is it pos- 
sible for these plants to grow in the dark? Why are they classed as sapro- 
phytes? Under what conditions are these forms beneficial? Under what 
conditions are they injurious to, man? Explain the part (if any) that each of 
the following processes has played in the life of these saprophytes: photo- 
synthesis, diffusion, osmosis, respiration. 

The use of saprophytes in the preparation of food. The yeast 
plant is a very small single-celled plant which reproduces by send- 
ing out buds which gradually are pinched off and become ne\\ 
yeast plants. Yeast cells grow in a sugar solution, giving off 
carbon dioxide gas and producing alcohol. This property of yeast 
cells is taken advantage of in bread-making. Dough containing 
yeast plants is kept at a temperature suitable for their growth. 
As the yeast cells grow, bubbles of carbon dioxide are formed 
throughout the dough. The bubbles cause the dough to rise. 
When this is baked, the alcohol escapes and the bubbles remain 
as holes in the bread, making it light. What is the source of the 
gas that forms the bubbles in bread dough? 

Bacteria and molds are used in the manufacture of dairy prod- 
ucts. Butter may be made either from cream as it is separated 
from the milk or from cream that has been allowed to sour. Sweet 
Bream gives butter which lacks the desired butter flavor and the 


butter soon becomes rancid. Sour-cream butter is the common 
type. The cream is allowed to sour from the action of lactic-acid 
bacteria. Country butter was formerly made from cream which 
soured naturally. Now most of the butter is produced in cream- 
eries where the cream is pasteurized to destroy wild bacteria 
present and then pure cultures of the lactic-acid bacteria are 
added. Under these conditions the flavor of the butter can be 

In the ripening of cheese, the desired flavor is produced by 
bacteria and molds, the particular flavor depending upon the type 
of organism present. 

Bacteria are also involved in the production of vinegar and in 
the making of sauerkraut. In a similar way the farmer uses a 
silo to preserve, by means of the acid formed, large quantities of 
food (silage) to be fed to cattle in the winter. 

Exercise 56. How can we test for the presence of carbon dioxide? Fit 

into a bottle a two-hole rubber stopper with funnel tube and with delivery tube 
extending into a test tube of lime water. Dissolve a little cream of tartar in 
water and pour the solution into the bottle through the funnel tube. Add a 
solution of baking soda. Carbon dioxide is released from the baking soda by 
action of the cream of tartar. It changes the appearance of the lime water 
from clear to milky as the gas bubbles through it. This is a test for carbon 

Exercise 67. What is the effect of yeasts on a sugar solution? Using the 
apparatus of Exercise 56, put a solution of one part molasses in nine parts of 
water into the bottle. Pulverize a small portion of a dry yeast cake and add to 
the molasses solution in the bottle. Let stand in a warm place for twenty-four 
hours with the delivery tube extending into the tube of lime water. Is there 
any evidence of the evolution of a gas in the bottle? Is the gas carbon dioxide? 
Note the odor of the solution in the bottle. The yeast cells produce an enzyme 
known as zymase. Sugar is taken into the yeast cell where the zymase causes a 
decomposition of the sugar with the production of alcohol and carbon dioxide. 

Suggested activity. How is yeast used in bread-making? Find out at 
home or from a baker how yeast is used in bread-making. What are some of 
the conditions necessary for success in making bread with yeast? 

Exercise 68. What is the nature of yeast cells? Place a drop of the liquid 
from the bottle used in Exercise 57 on a slide. Place over it a cover-glass, and 
examine under the low power of the microscope. The numerous small bodies 
are yeast cells. Using the high power, note the shape and structure. The 
knobs on some of the cells are buds which develop and finally separate, pro- 
ducing new cells. This process, which is the ordinary method of reproduc- 
tion of yeast, is known as budding. 


Food in which saprophytic organisms are growing may or may 
not be poisonous, but the presence of molds on preserved food is 
always a danger sign indicating that injurious organisms may 
be present. 

There are two types of poisoning which may result from eating 
contaminated foods: (1) the so-called ptomaine poisoning, and 
(2) botulinus poisoning. In the decomposition by bacteria of fish, 
meats, etc., which are mainly protein, poisonous substances are 
often formed which are very toxic when taken into the human 
digestive tract. These, together with the living bacteria present, 
may cause serious illness. In the second type of poisoning, an 
organism which is hard to kill because it forms spores may be sealed 
up in a can with food, and if the can is not sterilized by heat the 
bacteria may grow and reproduce in the food. This organism grows 
readily in the absence of air, and it produces a substance which is 
very poisonous but is easily destroyed by heating. Between the 
years 1919 to 1924 there was an outbreak of food poisoning in vari- 
ous parts of the United States. Canned ripe olives caused most of 
the trouble. As ripe olives are eaten without being cooked, the 
toxin was taken into the digestive tract. It is well to remember 
that clean, fresh, sound food will not cause botulism, and preserved 
foods freshly heated to the boiling point will not cause botulism. 
Ordinarily there is no danger in eating factory-canned foods as 
they are subjected in the can to high temperatures sufficient to kill 
spores as well as all vegetative bacteria. 

Food showing any signs of decomposition evident by appear- 
ance, odor, or formation of gas should be destroyed. It is unsafe 
to taste food which shows any of these signs of spoilage. 

How may disease be spread by milk? Milk, a balanced food 
for man, is also a suitable food for bacteria. The souring of milk 
is one of the first evidences that bacteria are growing in it. Disease 
bacteria from the cow herself or from infected persons may be 
present as well as the lactic-acid organism which causes the souring. 
Diseases which may be spread by milk are tuberculosis (to which 
the cow is subject as well as human beings) and diseases of human 
origin as typhoid fever, scarlet fever, septic sore throat, and dysen- 
tery. In what ways may harmful bacteria get into milk? 

Boards of health of our larger cities set standards for sanitary 



production of the supplies of milk which go into the cities. They 
also maintain an inspection service to see that the standards are 
met by the dairymen. One of the first requirements is healthy 
cows. They must pass the tuberculin test a test to determine 
the absence of any tuberculosis infection. Stables must be well 
lighted and kept clean. 

Pasteurization of milk. Heat not only reduces the bacterial 
content, but it may also cause changes in the protein food sub- 
stances in the milk. Pasteurization is a process in which the milk 
is kept at a temperature of 
65 C. for 20 minutes. This 
heat is sufficient to kill 95 to 
99 per cent of the micro- 
organisms, and the changes in 
the proteins caused by heat 
are reduced to a minimum. 
Give two reasons why city milk 
supplies should be pasteurized. 

Certified milk is produced 
under the strictest require- 
ments for cleanliness and is 
thus kept relatively free from 
bacteria. The cows are kept 
clean; they are washed before 
being milked ; the milker must 
wash his hands before milking 
each cow, and the milk must 
be cooled quickly. It is not 
heated but is sold as raw milk. 

It costs money to observe all the extra precautions required so 
that when ordinary pasteurized milk may sell for ten cents a 
quart, certified milk may sell for twenty cents a quart. 

The preservation of foods. During certain seasons of the 
year foods are plentiful in the fresh state. With our modern 
methods of packing and transportation we really have a wide 
choice of fresh fruits and vegetables the year round. However, 
during the winter, certain fresh foods are high priced and man 
has learned how to preserve foods when they are plentiful so that 

FIG. 36. A milk pasteurizer. The 
steam and refrigerator pipes with 
which it is connected are shown. (From 
California Agricultural Experiment 
Station Circular 319.) 


an abundance of a wide variety of foods is available throughout 
the year at moderate prices. 

The bacteria, yeasts, and molds thrive on our foods; they can 
be considered our rivals as they will surely spoil foods for our use 
if they can get to them first. The methods of preservation some- 
times change the flavor of foods, but we have learned to enjoy 
them and usually the nutritive value is not impaired. 

Methods of coping with the saprophytic menace to our food 
supply are concerned with keeping the micro-organisms out of the 
food by setting up conditions which make it impossible for them 
to grow in the food materials. 

One of the oldest and simplest of the methods of food preserva- 
tion is drying. Bacteria, yeasts, and molds require water for 
growth, and it is impossible for them even to begin growth in 
dried meats, fruits, and vegetables so long as they are kept dry. 
Drying reduces the amount of water in the food substances; if 
bacteria or other saprophytes come in contact with them they can 
not live, since the water molecules are in greater Concentration 
in the cells of the plant than in the food, and water passes from 
the plant cell and causes its death. 

The use of salt and sugar in the preservation of foods is a drying 
process, and the destruction of bacteria and other plants in this 
type of food preservation is really the result of osmosis. Water 
passes from the cells of the invading organism into the food, and 
the plants are killed by the loss of water. 

Sometimes chemicals, such as benzoate of soda and salycilic 
acid, are used in food preservation, but these are considered 
injurious to man to a greater or less degree. Meat and fish hung 
near a smoldering wood fire absorb certain acids from the smoke 
which preserve the foods without being markedly injurious. 
Smoking has long been used as a method of preservation of meat 
and fish. 

Methods of food preservation by canning were introduced 
early in the nineteenth century. These have proved far superior 
to the older methods. Bacteria do not thrive in the acid juices 
of fruits. Boiling the fruit for a few minutes kills the yeasts and 
molds, and if the heated materials are sealed in the can while they 
are hot, they will usually keep indefinitely in perfect condition 


This type of canning is easily done at home. For peas, beans, corn, 
and meats, the problem becomes more complex, as the spores of 
bacteria which may be present in these non-acid substances are not 
killed by simple boiling and, sealed in the can, the spores germinate 
and the bacteria reproduce rapidly in the abundant food supply 
with the result that the food is spoiled for human use. The com- 
mercial method of canning these substances consists in placing 
the hot materials in cans, sealing, and heating them under steam 
pressure to a temperature of 240 F. for 40 to 60 minutes. This 
treatment kills the very resistant spores of bacteria on the food 
and the food can not spoil. 

In home canning the cold-pack method is usually followed. 
The cans of food are sterilized either by steaming in an ordinary 
steam cooker for 3 hours or by heating in a pressure cooker under 
a pressure of 5 to 10 pounds of steam for one hour. 

One of the commonest methods of keeping the food from 
spoiling is by keeping it cold. Although the growth of molds and 
bacteria is not entirely stopped by low temperatures, foods can be 
kept very much longer if they are kept cold. Ordinary low tem- 
peratures used in refrigeration can not be depended upon to pre- 
vent food spoilage indefinitely. Mechanical refrigeration has 
added much to the convenience and safety of preserving foods 
temporarily by keeping them cold. Foods, especially fish and 
meats, should be used promptly after removal from cold storage. 

Exercise 59. Putrefaction of food materials. Place in a series of test 
tubes with a little water small bits of food substances, as meat, potato, bread, 
sugar, starch, flour, beans, and corn meal. Plug with a cotton stopper and 
leave in a warm place for a few days. What evidence is there that putrefaction 
has taken place? Is the result the same in all the tubes? What foods, if any, 
have not putrefied? 

Exercise 60. Will dry foods putrefy? Place in a series of test tubes dry 
food substances as beans, flour, corn meal, rolled oats, dried beef. Plug with 
a cotton stopper and examine after they have remained in a warm place for 
several days. Determine whether there has been any putrefaction. What is 
the relation of moisture to putrefaction? 

Exercise 61. What is the effect of heat on putrefaction? Put bits of 
meat, boiled beans, bread, milk, etc., into a series of test tubes with a little 
water. Plug with cotton stoppers and heat in a pressure cooker at 10 pounds 
pressure for 15 minutes, or in steam in a closed vessel for one hour. Set the 
tubes aside in a warm place and examine after a week. Answer the question 
of the exercise. 


Saprophytes in the soil. Under suitable conditions of moisture 
and temperature, a fertile soil will continue to support a vigorous 
growth of plant life year after year, and this has been continuing 
for hundreds of years. Even large trees have reached maturity 
and fallen, the wood gradually decomposing and dropping to 
pieces to become a part of the soil. Leaves, as they fall from the 
trees each autumn, do not continue to accumulate but gradually 
disappear, their partly decomposed structures forming humus in 
the upper layers of soil. We might expect that, while the plants 
are taking raw materials from the soil season after season, as time 
goes on it would gradually become exhausted so that it would be 
less able to support plant growth. This, however, is not the case. 
In fact, the opposite is true ; the soil actually increases in fertility. 

Bacteria and molds cause decay. The succession of plant 
growth on soil is made possible by the action of microscopic plant 
life. Many different micro-organisms in the soil bring about the 
decay of plant and animal materials. 

Some forms of saprophytes destroy the cellulose which makes 
up the walls of the plant cell. These are among the most impor- 
tant destroyers of plant material. It is interesting to note that 
bacteria of this class are present in the intestinal tract of herbivo- 
rous animals, such as cattle and sheep, and make digestion of coarse 
Jhay and fodder possible. What different things must happen to 
-a piece of wood before the substances of which it is composed can 
be used by other plants? 

The cycle of nitrogen in nature (Fig. 37). One of the most 
important of the elements used by plants is nitrogen. All protein 
substances contain this element along with carbon, hydrogen, 
oxygen, and small amounts of other elements. Although nitrogen 
makes up four-fifths of the atmosphere by volume, its compounds 
are the most expensive of the commercial fertilizers. Green 
plants can not make direct use of any of this necessary element in 
the form in which it occurs in the air. Nitrogen is set free and 
lost to plants in decaying protein materials, and large quantities 
of its compounds are lost as sewage. It was formerly thought that 
when the nitrate beds of Chile became exhausted there would be 
famine on the earth for the want of raw materials containing nitro- 
gen for plant growth. It has been found that certain micro- 



organisms in the soil have the power to bring about a combination 
of nitrogen of the air with elements in the soil, forming compounds 

Living things use food 
produced by green plants, 
use oxygen and give off 
carbon -dioxide. 


Carbon Dioxide 

Plant and 
Animal Remains 

Food of Animals 

Green plants use 
carbon-dioxide and give off 
oxygen in food-making. 
Nitrogen salts are used 
in making proteins. 

Nitrogen of 
S* the Air "\ 

Animal Wastes 

5acteria of decay act 
on the dead bodies of 
plants and animals and 
on animal uuastes forming 

Bacteria on legumes and 
other nitrogen -fixing bacteria 
take nitrogen from the air and 
form compounds used by plants. 


Denitrifying bacteria break 
doom useful nitrogen compounds 
setting nitrogen free. 



Nitrifying bacteria act 
on ammonia , forming 
nitrates vjuhich green 
plants can use. 

FIG. 37. Diagram showing the relation of living things in nature. In what 
ways do other living things depend on green plants? How are green plants 
dependent on bacteria? What part do the nitrogen-fixing bacteria play in 
nature? Trace the cycle of carbon in the diagram. Trace the cycle of 
oxygen as it appears in the diagram. Trace the cycle of nitrogen from the 
time it is a part of the food of animals, through the different processes until 
it is again a part of the food of animals. 


which plants can use. In this way the supply of nitrogen in the 
soil may be kept practically constant. 

There are two principal groups of nitrogen-fixing bacteria: 
(1) those which live free in the soil, and (2) those which are asso- 
ciated with the roots of legumes, as red clover and alfalfa. The 
latter are examples of symbionts. At the site of their invasion 
of the root tissues, galls or tubercles form. The legume absorbs 
the raw materials, salts and water from the soil, and the bac- 
teria make use of certain of these materials. The bacteria combine 
nitrogen of the air with materials furnished by the legume to form 
proteins for themselves; and the legume also makes direct or 
indirect use of these. In this relationship the two organisms living 
together are mutually beneficial. So dependent are the legumes 
on the tubercle bacilli that, if the necessary bacteria are not present 
in the soil, the seed of alfalfa or the soil in which the seed is planted 
must be inoculated; that is, the necessary form of bacteria must 
be introduced before the alfalfa can be grown successfully. 

It has long been known that the same crop can not be grown 
indefinitely year after year on the same soil. The farmer may 
grow corn followed by wheat, and with the wheat he may sow 
clover. The clover crop may be used for hay, but roots and por- 
tions of the stem rich in nitrogen compounds are left in the soil. 
When these materials decay, nitrogen is added to the soil. The 
system in which different crops are grown successively is known 
as crop rotation, and every well-planned crop rotation scheme should 
include a legume every two or three years. Give two reasons why 
a certain crop should not be grown on the same soil year after year. 

The nitrogen-fixing bacteria require well-drained soil which 
contains organic matter. If it is highly acid, lime must be added 
to reduce the acidity as these bacteria do not grow well in an acid 

In our account of the nitrogen cycle we may start with the 
complex plant and animal proteins. These are broken down by 
many different soil organisms into simpler and simpler substances 
until the final products can be absorbed by green plants as raw 
materials out of which more proteins are made. 

The bacteria of decay cause the decomposition of complex 
proteins in plant and animal remains and the formation of ammo- 


nia. Resulting ammonia compounds are changed by other bac- 
teria into nitrites. Nitrities are changed by still other forms into 
nitrates. Nitrates in solution may be absorbed by the green 
plant, where they unite with carbohydrates to form amino acids. 
Amino acids are combined by plants to form plant proteins. 
Plant proteins are eaten by animals, and animal proteins are 
formed. The dead bodies of animals and plants are attacked by 
bacteria, and a new cycle is begun. 

It should be noted that not all the bacteria involved in the proc- 
esses of the nitrogen cycle are saprophytes. The ammonia- 
formers derive energy from the decomposition of dead plant and 
animal materials; thus, they are true saprophytes. The nitrate- 
and nitrite-formers are as truly independent as green plants since 
their sources of energy are chemical substances, the ammonium 
compounds. The nitrogen-fixing forms which live free in the soil 
require carbohydrate foods produced by green plants for their 
source of energy, and therefore they are saprophytes. The nitro- 
gen-fixing forms which live in tubercles on legume roots derive 
their energy from the carbohydrates and other food supplies of the 
cells of the living host plants, and hence they are parasites. 

Exercise 62. Root tubercles. Examine roots of a clover plant for evi- 
dences of the presence of bacteria. Crush one of the tubercles, and note the 
milky contents; this material contains the nitrogen-fixing bacteria. Where 
do they get their food? What do they do for the plant? What does the 
legume plant do for them? Are they saprophytes or parasites? Show that 
the legume plant and its nitrogen-fixing bacteria are symbionts. Of what 
importance are these forms in nature? Of what importance are they in 
agriculture? In what way does a clover crop add to soil fertility? Write a 
paragraph showing what would be the result if all the nitrogen-fixing bacteria 
should cease to function. 

Problem 3. How do parasitic plants cause disease in 

Comparatively few of the very large number of different kinds of 
fungi that have been identified have been found to cause disease 
in animals. Although a number of molds cause disease, such as 
ring- worm and athlete's foot, yet most of the pathogenic fungi are 


Leeuwenhoek first discovered bacteria late in the seventeenth 
century. It was not until two hundred years later that they were 
shown to be the cause of disease. In a series of epoch-making 
experiments and in the face of much opposition, Robert Koch, 
around 1876, showed that anthrax in sheep is caused by a rod- 
shaped bacterium. He later showed that tuberculosis is caused 
by another rod-shaped form. Louis Pasteur entered into the 
study of anthrax in France with a view to finding some method 
of preventing or curing the disease. His work resulted in a method 
of preventing anthrax in animals by vaccination. This early 
work was important as it not only met the immediate need of a 
method of preventing anthrax but it also laid the foundation for 
the development of our later knowledge along the lines of disease 

Many pathogenic bacteria produce poisons in the body of the 
animal which is their host. These poisons, known as toxins, cause 
the symptoms of the disease. It has been found that substances 
which counteract the toxins are developed in the body of the 
infected animal. These are known as antitoxins. 

The resistance of a body to disease bacteria is known as immu- 
nity. For some unknown reason, some individuals have a natural 
immunity to a certain disease. They do not easily contract the 
disease. A person who has had diphtheria is not likely to have 
the disease a second time. He is protected by substances in the 
blood which are formed as a reaction to the effects of the toxins. 
This immunity is known as acquired immunity. Investigators 
have learned that diphtheria antitoxins, developed in the blood of 
a horse, can be used safely in protecting children against diph- 
theria by providing an immunity. Vaccination is practiced by 
physicians to protect against certain diseases by causing the 
patient to have a mild form of the disease, with development of an 
immunity which protects against the more serious form of the 

The first successful vaccination was performed by Louis 
Pasteur in the prevention of fowl cholera in chickens. He later 
successfully vaccinated sheep against anthrax. Pasteur's name 
has gone down in history as one of the greatest benefactors of the 
race. He was able to show from his knowledge gained in the 


study of dangerous bacteria how bacteria could be kept out of 
wounds. Keeping a wound free from bacteria is known as asepsis 
(without disease). Lister had already shown how bacteria could 
be destroyed by the use of chemicals. The term applied to this 
method of preventing infection is antisepsis (against disease). 
Name five diseases of man which are caused by bacteria. 

Problem 4, How may bacteria and molds be studied in the 


In growing bacteria and molds for study, it is necessary to 
furnish food materials for the plants and keep the temperature 
and moisture conditions suitable. It is also necessary to be cer- 
tain in most experiments that the food materials and glassware 
used are free from bacteria and mold life at the beginning of the 
experiment and that they be kept so throughout its progress. 
Glass bottles or test tubes with stoppers made of wads of cotton 
can be sterilized by heating in an oven until the cotton is slightly 
browned. Culture media containing the food for the plants can 
be sterilized by heating the cotton-stoppered bottles or flasks of 
the material in a pressure cooker with steam at a pressure of 
15 pounds. An ordinary kitchen steam cooker may be used if a 
pressure cooker is not available. Heating in steam in such a 
vessel for 30 minutes may not destroy all the spores that may be 
in the culture material, but this method is sufficiently efficient 
for use in experiments in high-school classes. Conn's Bacteria, 
Yeasts, and Molds in the Home gives detailed directions for the 
preparation of media and the doing of many interesting and instruc- 
tive experiments^ suit able for the work of high-school classes. 

Exercise 63. Bacteria in water. Crowd leaves and stems of water plants 
into a jar of water and set aside to let the materials decay. Note changes in 
the appearance of the liquid as decomposition proceeds. Mount on a glass 
slide under a cover-glass some of the cloudy liquid as it develops. Examine 
under the high power of the microscope with most of the light shut off. What 
evidence of life do you see? Does decay give rise to the bacteria, or do the 
bacteria cause the process of decay? What happens to the solid materials as 
decay goes on? What is in the water that was not there before decay started? 
Why is pure water less likely to have living bacteria in it than water polluted 
with sewage? Why is water from a deep well less likely to contain large 
numbers of bacteria than water from a river? 


Suggested activity. Make a study of the water supply of your community: 
is it surface water, as that from a stream, pond, or lake; or is it ground water, 
as that from a deep or a shallow well? Is it likely to have sewage in it and thus 
carry dangerous as well as other forms of bacteria? Present a report of your 
study to the class. 

Write a report on methods used in your community to make the water 
supply safe for drinking purposes. 

Exercise 64. Bacteria in milk. Put in each of six large-mouth, half-pint 
bottles one-third of a pint of raw milk that has not been heated. Close each 
bottle with a wad of cotton. Heat three of the bottles in a steam cooker or 
other closed vessel with water for half an hour. Label the bottles to identify 
them, and set aside with the unheated bottles in a moderately warm place. 
Note the appearance of each of the bottles daily for a week. At the end of 
that time remove the stoppers and note the odor of the milk in each bottle. 
Account for any differences. Why is milk a good medium for bacteria? 
Under what conditions will milk sour? What is the danger In washing 
bottles, cans, and other utensils used in handling milk in water contaminated 
by sewage wastes? What is the danger in allowing a person harboring disease 
germs, as a carrier or himself having the disease, to handle the milk? 

Suggested activities. Write an account of the methods used to keep your 
milk supply free from dangerous bacteria. 
Justify the expense of city milk inspection. 

Bacteria in the air. Wherever there is dust in the air there are 
bacteria. Bacteria ride on particles of solid material of which dust is 
composed. It is true, not every speck of dust has its passenger; 
but wherever there is dust you can be certain there are bacteria. 
Most of them are harmless ; others are helpful, falling upon organic 
waste materials and causing them to decay; still others get into 
exposed foods and cause them to spoil, and we breathe some into 
our throat and lungs which may cause inflammation. On a 
mountain top there are few bacteria, but in crowded centers of 
population they are everywhere in great numbers. 

Exercise 65. Bacteria in the air. Wash and dry thoroughly petri dishes 
with covers. Place them in an oven and gradually raise the temperature to 
moderate heat. Turn off the heat after half an hour, and when they are cool 
wrap each dish without removing the cover in paper for protection from dust. 
Prepare nutrient Bacto agar as directed, and while it is still hot remove care- 
fully the cover of a dish and pour into it enough of the liquid to cover the 
bottom of the glass. Replace the cover immediately and repeat the process 
until the required number of dishes has been prepared. When cool, the 
nutrient material which contains the food for bacteria should be a jelly. 


Place a dish on a table in the laboratory and remove the cover, exposing the 
jelly for five minutes. Replace the cover and set the dish in a warm place. 
Examine daily. The spots which appear on the agar are colonies of bacteria, 
each having started with a single bacterium which fell upon the jelly at that 
point while the surface was exposed in the room. Test for the presence of 
bacteria in the air of the assembly hall, that of the corridor when classes 
are passing, of the living room at home, and of a park. Which is most effect- 
ive in dusting, from a sanitary point of view a feather duster, a dry 
cloth, or a damp cloth? Why are outdoor diversions to be preferred, in 
general, to spending leisure time indoors? 

Suggested activity. 
Devise an experiment us- 
ing petri dishes of nutrient 
agar to show that it is not 
only bad form to sneeze or 
cough in public without 
covering the nose and 
mouth with a handker- 
chief but that the prac- 
tice is, besides, decidedly 

Exercise 66. To collect 
bread mold. Moisten a 
thin slice of rye bread 
with prune juice and fit it 
into a petri dish. Leave 
uncovered in the labor- FIG. 38. Drawing of a portion of a mycelium of 

bread mold (Rhizopus nigricans). The mold 
spreads over the surface of the food material by 
means of hyphae called stolons. Other hyphae, 
known as rhizoids, are special absorbing organs 
of the plant. The black knobs (white when 
young) are sporangia which bear asexual spores. 
They are supported by hyphae called sporangio- 
phores (spore-sac-bearers). 

atory for fifteen minutes, 
then cover and set away 
in a moderately warm 
place for two or three days. 
Molds of various kinds, 
including bread mold, will 
probably be found growing 
on the bread from spores 

collected from the air of 

the room. Dark portions of the molds collected in this way are usually growths 
of bread mold. These can be picked out with a pair of fine-pointed forceps 
for use in starting a pure culture of bread mold in the following exercise. 

Exercise 67. What is the nature of bread mold? Fit thin slices of rye 
bread into petri dishes. Cut a IJ^-inch square of the bread out of the center 
of each dish, leaving a window through which the growing mold can be studied. 
Moisten the bread in the petri dish with prune juice, and transfer some of the 
mold of the previous exercise to the bread on two opposite sides of the window. 
Replace the cover and set the dishes away in a moderately warm place. Examine 


frequently after the first twenty-four hours without removing the cover. As 
the mold grows out from the bread and across the window, it can be studied 
with a lens or by placing the dish bottom side up under the microscope, using 
the low power. Placing a few drops of prune juice on the portion of mold in 
the window to be studied, covering this with a cover-slip will permit a better 
view of the growing mold. Note that the separate threads (hyphae) are with- 
out cross walls and that they branch repeatedly. The whole mass of hyphae is 
a mycelium. The hyphae that grow along the surface of the bread are known 
as stolons. The knobs, at first white and later black, on the ends of hyphae 
extending out from the stolons are sporangia. These bear spores which ripen 
and are blown about by the wind, some of them reaching new sources of food 
where they may germinate and produce other mold plants. At the base of the 
sporangium-bearing hyphae are root-like hyphae (rhizoids) fitted by structure 
for going down into the medium (the bread) on which the mold is growing and 
from which it is absorbing food. 

Write a summary giving the r61e of each of the three types of hyphae of 
bread mold which you have seen. In what two ways does bread mold spread 
to new sources of food? In what respects are spores better for distributing the 
plant than fragments of the mycelium? 

Gametic reproduction. Under certain conditions a fourth 
type of hypha is produced by bread mold. It has been demon- 
strated that there are different, distinct strains of the plant. If a 
stolon of one strain grows near the stolon of another strain of the 
plant, special hyphae are sent out from the two stolons. These 
approach each other. When they come in contact, end to end, 
a special cell is formed in each. These cells fuse, making a single 
cell which forms about itself a heavy wall which protects the living 
material inside, and carries it through extended periods during 
which conditions are not favorable for growth of bread mold. 
The two cells which unite are called gametes, and the single cell 
formed from their union is a second kind of spore, a zygospore. 
Under favorable conditions, after a dormant period, the zygospore 
germinates and produces another plant. The type of reproduc- 
tion involving gametes is known as gametic or sexual reproduction. 
Note the four types of hyphae shown in Fig. 39. 

Exercise 68. Gametic reproduction in bread mold. To grow zygospores 
of bread mold. It is necessary to have a culture of a " plus " strain of 
Rhizopus nigricans and a culture of a " minus " strain of the mold. These 
can be bought from dealers in biological supplies. Prepare a petri dish with 
rye bread and prune juice as in Exercise 67. Using fine-pointed forceps, 
inoculate, with a streak of the " plus " strain and a streak of the " minus " 


When asexual spores from 
two strains of bread mold 
qermmate near each other 
qametes are formed 

When food is plentiful 
asexual spores man reproduce 
the plants aver and over aqain 
This is the usual method of 

Each plant sends out a 
peculiar Kind of hvjpha 

Gametes develop where 
the hi|phae meet 

Asexual spores 
qermmate and form two 
strains of mold plants 

The zqqote qerminates 
and two kinds of asexual 
spores are formed 

Two qametes umte to 
form a z.t|qote, a restmq spore 

FIG. 39. Life cycle of bread mold (Rhizopux m^ncans). The usual method 
of reproduction of bread mold is by asexual spores. Under what conditions 
in nature may zygotes be formed? How could bread mold be distributed 
to new sources of food? By what two methods can a piece of bread be com- 
pletely covered with mold as the result of the germination of a single asexual 
spore? What is a possible r61e of zygotes in the life cycle of bread mold? 
The union of two similar gametes resulting in the production of a zygote 
as it occurs in bread mold is a simple type of gametic reproduction known 

as conjugation. 


strain, the surface of the bread on opposite sides of the window. Allow growth 
to continue for a week. Look for stages in the forming of zygospores in the 
window between the two streaks. Bits of this material may be easily lifted out 
and studied under the microscope. 

How can bread mold use the nutrient materials on which it 
grows? From the action of enzymes produced by the rhizoids, 
complex food materials in the bread are broken down into simpler 
substances. In these simpler forms the nutrient materials can 
diffuse through the membranes of the rhizoids. Inside the plant 
these foods pass along the hyphae by diffusion. By oxidation of 
nutrient materials, energy is released. Thus the plant can carry 
on the necessary life processes. By assimilation, some of the 
nutrient materials become a part of the protoplasm, making it 
possible for the plant to grow. 

Problem 5. How do parasitic plants cause plant diseases? 

We have found that bread mold may cause the gradual break- 
ing down of the chemical compounds which are found in bread or 
other nutrient substances which are non-living organic materials. 
Rhizopus nigricans and related molds may also become parasitic, 
invading living plant tissues and causing plant diseases, among 
which are certain rots of sweet potatoes, strawberries, apples, 
raspberries, and other plant products. Spores of Rhizopus are 
widely distributed, and every precaution must be taken by growers 
to avoid conditions which would be favorable for their invasion 
and growth in the food materials. As an example, sweet potatoes 
must not be bruised in handling, and they must be carefully dried 
and aired during the time that they are apt to heat and collect 
moisture in storage. Wrapping apples in paper tends to keep 
them dry and protects against invasion by Rhizopus, thus prevent- 
ing apple rot from this cause. 

Plant diseases caused by bacteria. The plant juices form 
favorable nutritive material for many bacteria. An acid condition 
of the sap may keep them out, as bacteria require a slightly 
alkaline or neutral medium. The juices of most plants, however, 
are suitable for the growth of these organisms. Bacteria may 
enter the host through wounds, or through openings, as stomata 



and lenticels. The bacteria may cause injury to the host by invad- 
ing the water-conducting vessels of the plant and cutting off the 
water supply, or by destroying soft tissues of the plant and pro- 
ducing blight or rot, or by stimulating the cells of the host to pro- 
duce abnormal growths as knots or galls. 

Fire blight. Where pears, apples, and quinces are grown it is 
quite common for branches of trees in full leaf to turn brown and 

FIG. 40. The large galls or warts are the result of infection by bacteria. 
The disease is known as crown-gall. 

die. The disease causing this symptom is fire blight, a serious 
bacterial disease. It may attack the blossoms, causing blossom 
blight, or the leaves, causing leaf blight. It may also affect the 
twigs, larger branches, or main trunk of the tree. It affects peat 
trees more than any other and is sometimes called pear blight, 



The cells of the soft tissues are invaded and killed by the bacteria. 
Measures for dealing with fire blight include control of insects, such 
as plant lice and other insects which carry the bacteria; cutting 
out infected parts of trees; and selection of varieties of fruit which 
are resistant to attack. 

Diseases due to rust fungi. The common name, rust, of this 
group is suggested by the colored spores which are conspicuous 
in certain stages of the development of the parasite. The rusts 

cause disease in nearly all groups of 
economic plants: grains, grasses, or- 
chard trees, garden vegetables, and 
forest trees. During the World War, 
when methods of greater food produc- 
tion and conservation were studied 
and carried out, a systematic attempt 
was made to eradicate wheat rust by 
ridding the country of its alternate 
host, the common barberry. 

Stem rust of wheat. This is some- 
times called black rust because of 
the black blotches which are found 
on the stem of wheat in certain stages 
of the disease. When the Chicago 
Board of Trade gets reports of extensive 
invasion of the wheat fields by black 
stem rust, there is usually an immediate 
rise of price of wheat. Why? The 
effect of rust may be slight, or almost a 
complete failure may result, depending 

upon the severity and time of attack. The damage to the host is 
caused by the loss of the food appropriated by the parasite and 
by the loss of water at the points where the epidermis is ruptured 
by the rust plant. 

Life cycle of stem rust. A convenient place from which to 
start is the germination of the teliospore, the two-celled resting 
spore of the parasite, by means of which it goes through the winter. 
In the spring, under favorable conditions, each of the two cells of 
the teliospore gives rise to a short hypha from which four sporidia 

FIG. 41. Germinating rust 

spores shown penetrating the 

tissues of a leaf by way of 

the stoma. 



(early spring spores) are loosed and blown away by the wind. 
These sporidia can not infect the wheat, and are lost unless they 
fall upon a leaf of the common barberry. The infection of the 
barberry leaf results in the production of aeciospores (late spring 
spores) which can not germinate on the barberry, but if blown to 
susceptible wheat, germinate and produce an infection. Reddish 
oblong spots, uredinia, soon appear with the production of large 

FIG. 42. A portion of a wheat stem 
infested with the black stem rust. 
Observe the hyphae (threads) among 
the cells of the host, and the cluster 
of spores. These are the one-celled 
summer spores (urediniospores). 

FIG. 43. A portion of a wheat stem 
infested with the black stem rust. 
Observe the hyphae (threads) 
among the cells of the stem, and 
the cluster of spores. These are 
the two-celled winter or resting 
spores (teliospores). 

numbers of the oval one-celled red summer spores of the rust, 
urediniospores. These spores are scattered by the wind to other 
plants of wheat, and thus the fungus is spread extensively during 
the growing season. Later, the urediniospores may be replaced 
on the same mycelium by the black or chestnut-brown two-celled 
winter spores, the teliospores. These are not capable of germinat- 
ing at once but must pass through a dormant period. Thus they 



are the resting cell of the fungus. Of what advantage is it to 
wheat rust to produce teliospores? 

It is seen that the barberry is the intermediate host in the life 
cycle of stem rust of wheat when barberries are present, and the 
presence of the barberry is probably responsible for severe destruc- 
tive attacks of stem rust. However, urediniospores may over- 
winter under certain mild winter conditions and be spread directly 
to the new crop of wheat plants. 

The main methods of control of stem rust are (1) eradication 
of the common barberry, and (2) the selection and breeding of 
immune varieties of wheat. Denmark has eradicated the bar- 
berry, and black rust has 
almost disappeared. In 
Wales, where the barberry 
is still allowed to grow, 
there are serious losses of 
wheat each year from rust. 
Among resistant varieties 
of wheat, Kanred, a variety 
of winter wheat, and Black 
Persian, a spring strain, 
were found from extensive 
experiments, to be most 

Apple rust may be men- 
tioned as another rust 

which normally requires an intermediate host for the completion 
of the life cycle. The infection of the apple leaves and fruit results 
in the production of aeciospores. These are blown to cedar trees, 
where they produce infection which results in the so-called cedar 
apples. The cedar apples develop spore horns which produce 
teliospores. These germinate, forming sporidia which are blown 
away and infect the leaves and fruit of the apple with the rust. 
In regions where there are cedar trees to be protected, cedar rust 
can be prevented by eradication of apple trees. In regions where 
apple growing is profitable, apple rust can be prevented by de- 
struction of the cedar trees. 

Stem rust of wheat and apple rust are examples of diseases 

I'iG. 44. A large uush ot common barberry 

an alternate host for the black stem rust of 

wheat and other cereals. 


caused by fungi which have more than one host. This habit of 
life makes their control by man very difficult. Consequently they 
have become the worst enemies of certain crop plants. 

Plant diseases due to smut fungi. Many plants among the 
cultivated cereals are affected by smuts. The parts most fre- 
quently destroyed are the grains or the flowers. As a result, every 
year there are heavy losses of cereal crops due to smut. 

One of the most destructive diseases of wheat is stinking smut 
or bunt. Infection takes place in the young seedling stage, from 
spores carried on the seed. As a result of germination, infection 
threads are produced which penetrate the seedling and reach the 
growing point of the host. The fungus remains inside the host 
and gives little or no indication of its presence until the head of 
wheat emerges. Infected plants produce smut balls instead of 
grain. Since the disease is caused by an internal parasite, the 
smut plant is living at the expense of the host, consuming food, 
destroying seed, and retarding the normal life processes of the host. 

Among other smuts which affect cereals are loose smut of 
wheat, loose smut of oats, and common corn smut. In the loose 
smuts of wheat and oats, the grains are destroyed. The smut 
which is formed in their place is loose and easily blown away. In 
corn smut, the usual symptom is the formation of small or large 
tumors on various aerial parts of the corn plant. At first, these are 
whitish, but later they become black owing to the development of 
spores. The membrane covering the mass breaks, and the mil- 
lions of spores are released. The estimated annual loss in reduction 
of yield of the corn crop due to smut is 21 per cent. (See Fig. 5.) 

Suggested activity. Make a collection of dry plant parts showing effects 
of plant parasites. 

Problem 6. What are the principal groups of fungi? 

As we have seen, fungi may be classified on the basis of nutri- 
tion into two main groups: saprophytes, forms which use non- 
living organic food; and parasites, which obtain their energy and 
materials for growth from living hosts. We have also learned that 
some of the bacteria are autophytes, securing their energy from the 
oxidation of inorganic substances. 


Bacteria. The bacteria represent the simplest fungi, being very 
minute, strictly single-celled forms. 

Phycomycetes. The Phycomycetes or alga-like fungi usually 
have a mycelium composed of hyphae which are tubes and not 
made up of cells arranged end to end in filaments. In this respect 
they resemble certain algae, as Vaucheria. This group includes 
bread mold, certain blights, and many related forms. The late 
blight-fungus of the potato, a phycomycete, was responsible for the 
loss of the potato crop which resulted in the great Irish famine 
of 1845. 

The white mold appearing on injured gold fish and frequently 

FIG. 45. The reindeer lichen thrives in the tundra of the far north; 

on young fish in hatcheries is a water mold, one of the alga-like 
fungi. ' The hyphae grow into the flesh of the fish, usually resulting 
in its death. 

Ascomycetes. The Ascomycetes, or sac fungi, differ from the 
alga-like fungi in that they are made up of hyphae composed of 
separate cells. The common blue and green molds often seen 
growing on fruits are examples of this group. The blue and green 
molds are the most widely known of the molds which are destroyers 
of foods. Fruits, fresh and canned, bread, and ever* smoked 
meats are attacked. Molds of this group are used ir> the manufac- 



ture of certain types of cheese to give them the desired flavor. 
These molds illustrate the characteristic of molds in general in 
being able to secure food and water from concentrated sugar 
solutions such as jellies and preserves. This property of the molds 

FIG. 46. Mushrooms growing from an old stump of a tree. 

results from the fact that the solution in the vacuole of the mold 
cell is comparatively highly concentrated. 

Lichens. The greenish or grayish patches growing on rocks, 
trunks of trees, and sometimes on the ground, which are known as 
lichens, really represent two different plants living together. The 
two plants, one an alga made 
up of green spherical cells, 
and the other a fungus, live 
together in a symbiotic rela- 
tionship. They are mutually 
beneficial. The alga makes 
food for itself and for the 
fungus. The fungus holds 
moisture which is used by the 
alga in the manufacture of 

food. Most lichen fungi are ascomycetes, bearing spores in sacs 
(asci) formed in brown cups or fruiting bodies appearing on the 
upper surface of the flat body of the lichen. Lichens are usually 
distributed by fragments of the lichen body. 

FIG. 47. Indian pipe, a saprophytic 
non-green seed plant. 



A number of serious plant diseases are caused by parasitic 
Ascomycetes. Among these are peach leaf curl and brown rot of 
stone fruits. 

Basidiomycetes or basidium fungi. The character which gives 
this group its name is the production of club-shaped hyphae or 
basidia which bear the spores. The group includes important 

parasitic and saprophytic 
forms. The rusts and smuts 
which affect grains, and the 
fungi which rot timber, are 
among the parasitic forms. 
Familiar saprophytes of 
the group are mushrooms, 

FIG. 48. Dodder, a parasitic non-green 
seed plant. Growing on goldenrod stems. 

FIG. 49. Cross-section of alfalfa 
stem and lengthwise section of a 
portion of dodder stem which is 
attached to the alfalfa stem by 
two haustoria. Dodder is a para- 
sitic seed plant. 

puffballs, earth stars, and stinkhorns. The part of the plant which 
attracts attention is really the fruiting body; the structures related 
directly to nutrition are hidden away in the substratum. 

Aside from the rusts and smuts, most of the Basidiomycetes 
which are of economic importance are wood-destroying forms. 
Some of these are purely saprophytic, destroying posts, piling, 


timbers, etc., which are continually moist and in contact with air. 
Others enter living trees through wounds, as those made in pruning 
or by hail or lightning. Infected trees may be greatly weakened 
by root rot or heart rot. The parasites are unable to advance into 
the active sapwood, and a tree may seem healthy from outward 
appearance even though its trunk may be hollow from the dis- 
integration of the heartwood by wood-destroying organisms. 

How do non-green seed plants obtain food? A number of 
seed plants without chlorophyll attach themselves to the roots of 
green plants and thus secure food which takes the plant through 
the complete cycle of flower-bearing and seed-producing above 
ground. The broom rape family includes the Indian pipe, pine- 
sap, beech drops, and squaw root. Some of these plants are 
parasites growing in contact with roots of trees. Indian pipe and 
pinesap are saprophytic, non-green seed plants, getting their food 
from humus. 

One of the most familiar of the parasitic seed plants without 
chlorophyll is the common dodder. The dodders are very close 
relatives of the morning-glory and sweet potato, although their 
resemblance is not evident to the casual observer. They have 
undergone marked changes due to their parasitic life. The seed 
germinates in the soil as does that of other seed plants. If the 
young seedling comes in contact with a suitable host, which may 
be a weed or a crop plant, it twines about the adopted plant, send- 
ing absorptive organs called haustoria into the tissues of the host. 
Manufactured foods are thus taken from the tissues of the host 
plant directly into the tissues of the parasite. Dodders are of 
considerable importance in various parts of the country as destruc- 
tive parasites on clovers, alfalfa, sugar beets, and other cultivated 


1. What is the explanation for the fact that among seed plants we find an 
occasional white seedling? 

2. Explain why a variegated plant is more likely to go down in the struggle 
in competition with all-green plants. 

3. One use of food in animals is to supply a source of energy for physical 
exercise. Why do plants need an energy supply in the form of food? 

4. We find large trees and small trees growing together in a forest. Explain 


why the age of the forest can not be determined by counting the annual rings 
on the stumps of the largest trees. 

5. Are the bacteria of decay the farmer's friends or his enemies? 

6. Why are bruised fruits more likely to rot than sound fruits? 

7. Why is it that bread dough, while rising, must be neither hot nor cold? 

8. Why is the flavor of butter made from pasteurized cream more constant 
than that made from raw cream? 

9. Why should canned fruit or vegetables be discarded without tasting if 
the ends of the tins are bulged outward before opening? 

10. Explain why it is not safe to eat canned fruits which have mold on the 
top of the fruit, even though the molds are usually not poisonous. 

11. How does smoke preserve meats? 

12. Why is it that clover and other legumes do not grow well on acid soils? 

13. In what way does a clover crop add to soil fertility? 

14. In what respects may the widely distributed sweet clover be considered 
a beneficial weed? 

15. What is meant by the nitrogen cycle in nature? 

16. What is the carbon cycle? 

17. Show the importance of the wide distribution of bacteria on the earth. 

18. Explain why a deep well is usually better as a source of drinking water 
than a shallow well. 

19. How can mold spores which are on the surface of fruit be prevented from 
germinating and causing rot? 

20. Knowing what you do about molds, how would you store apples to pre- 
vent loss by rotting? 

21. Make a spore print of a mushroom by placing the cap of the mushroom 
on paper, gill side down. Why is it necessary that such large numbers of spores 
are produced? 

22. What are the disadvantages of the parasitic habit to the plant parasite 


Bacteria, Yeasts, and Molds in the Home, by H. W. CONN, revised edition 
by H. J. Conn, 1932. 320 p. Published by Ginn and Company. Should be 
available for reference throughout the study of the unit. Useful and inter- 
esting facts about non-green plants that every student should know. Appendix 
with many easy experiments that may be performed by students who are able 
to do extra work. 

Microbe Hunters, by PAUL DEKRUIF. Revised edition. Published by 
Harcourt, Brace, 1926. Biographies. Interesting account in story form of 
the lives and work of Koch, Leeuwenhoek, Pasteur and others who were 
pioneers in the field of the relation of bacteria to disease. 

Science in the Service of Health, by E. R. DOWNING. 320 p. Published 
by Longmans, Green and Company, 1930. Easy reading in the history of 
the discoveries concerning the relation of bacteria to disease. 


Man and Microbes, by S. BAYNE-JONES. Published by Williams and 
Wilkins Company, Baltimore, 1932. 128 p. An interesting written picture 
of Bacteriology. 

Soil and the Microbe, by S. A. WAKSMAN and R. L. STACKEY. Published 
by John Wiley and Sons, New York, 1931. 

Civilization and the Microbe, by ARTHUR I. KENDALL. Published by 
Houghton Mifflin Company, Boston, 1923. 231 pages. This book tells the 
story of the microbe in simple language understandable by the student of 
high-school age. It tells in a most interesting way what bacteria are, of the 
various forms of bacteria, of the temperature range of microbic life, of the 
nutrition of bacteria, of bacteria and the industries, of disintegration of animal 
and plant remains by bacteria, of diseases caused by bacteria. 

Who's Who Among the Microbes, by WM. H. and ANNA W. WILLIAMS. 
Published by the Century Company, New York, 1929. 302 pages, illustrated. 
The sketches in this book grew out of a series of radio talks on communicable 
diseases and their microbes. The authors have endeavored to describe 
"simply and accurately the most important facts known that help us determine 
how and why some microbes are harmful to man, others harmless and still 
others helpful." They also tell how man can use available knowledge to 
protect himself against harmful bacteria and utilize more fully the activities 
of the useful bacteria. 

Bacteria in Relation to Man, by JEAN BROADHURST, published by J. B. 
Lippincott Company, Philadelphia, 1925. 306 pages, 144 figures. A study- 
text in general microbiology, dealing with molds, bacteria, air, water, milk, 
soils, and human disease. At the end of chapters are excellent reference lists. 


When a plant or animal grows it becomes not only larger, but 
also different in the structure and relationship of its organs, and 
in its behavior. That is, growth involves more than simple 
enlargement. When an acorn grows into the mighty oak, the 
full-grown tree is not merely the enlargement of a miniature oak 
concealed within the acorn. The mature tree possesses tissues and 
organs that did not exist in the acorn. Moreover, it has certain 
activities not carried on by the young plant. And, too, growth 
usually involves a change in the chemical composition of the plant. 
Growth then has two phases: (1) increase in size, due to increase in 
number and size of cells, and (2) "a becoming-different," more tech- 
nically called differentiation. This latter phase usually means an 
increase in complexity. The marvelous feature of growth is this 
differentiation the development of a complex structure such as 
the oak tree with its roots, leaves, stems, flowers, fruits, and seeds, 
and the many different kinds of tissues which compose these vari- 
ous organs, from a few simple tissues in the acorn. But still 
more wonderful is the development of the miniature plant hidden 
within the seed from a single cell. The fundamental fact to keep 
in mind is that the adult plant is derived from a single cell. The 
beginning of every plant is a single cell. In seed plants this 
single cell is in a part of the flower called the pistil. 

Not all parts of the plant grow at the same rate. For example, 
early in the germination of a bean seed the young root grows more 
rapidly than the stem. When a flower bud opens there is, for a 
short period, a very rapid growth of the flower organs. 

Growth of a plant is the resultant of all its other activities. 
Every activity must progress normally if the plant is to grow 
properly. There must be absorption of mineral salts from the soil 
and of oxygen and carbon dioxide from the atmosphere ; food man- 




ufacture and respiration there must be; there must be adequate 
transportation of substances throughout the plant; and there must 
be food digestion and assimilation. Then we know that all these 
functions are influenced by certain environmental conditions such 
as temperature, moisture, light, the supply of raw materials for 
food manufacture, etc. The growth of a plant is, in many respects, 
like that of a boy or girl. Unless all functions of the body 



Foliage Leaves 

FIG. 50. Successive stages in the germination of the seed of the bean. (From 
Holman and Robbins in Elements of Botany.) 

respiration, digestion, assimilation, excretion, etc. are normal, 
and unless the environmental conditions which control these func- 
tions are suitable, the boy or girl does not grow as he or she should. 
Not all plants grow at the same rate. We recognize that it is 
the nature of some kinds of plants to grow more rapidly than 
others. For example, certain poplars attain a height of 15 to 20 
feet within a few years, whereas slow-growing oaks may require 


three or four times as many years to attain this height. Ordi- 
nary field corn grows to a height of 6 or 8 feet during the growing 
season, whereas certain dwarf varieties of corn under similar 
environmental conditions may attain a maximum height of not 
over 2 or 3 feet. We have learned that food is made in the chloro- 
phyll-bearing cells. Leaves are the principal food-manufacturing 
organs. Do you think that the total leaf area of a plant has an 
influence on the rate of growth of a plant? Do you think that 
temperature, light, moisture, fertility of the soil, and other environ- 
mental conditions influence the rate of growth? Explain. 

Problem 1. How do embryos grow? 

The young, undeveloped plant as it exists in the seed is called 
an embryo. It is a simple structure with relatively few organs. 
For that reason, we say it is undifferentiated. 

Exercise 69. Structure of embryo. Soak the seeds of beans, peas, squash, 
etc., in water. After they are softened, carefully remove the seed coats and 
separate the embryo. Each kind of embryo has a characteristic form, and 
certain organs. For example, in the bean embryo, notice the two large fleshy 
cotyledons, the very short, young root, the miniature stem, bearing two dimin- 
utive leaves. Compare the embryos of several different kinds of plants. 

Development of the embryo following fertilization. We are 
familiar with the fact that seeds are formed hi flowers. Seeds 
develop within a certain structure of the flower called the pistil. 
Long before the flower is open, there develops within its pistil one 
or more small masses of tissue, each of which is destined to become 
a seed. Among the cells of each mass of such tissue there is one cell, 
the so-called egg cell, which is destined to become the embryo 
but only after it is fertilized. Fertilization consists in the union 
of the egg cell with a cell which is formed within the pollen grain. 
In other words, fertilization involves the union of two living cells 
one developed in the pistil, the other developed in the anther. 
The embryo begins its life as a single cell the result of a union. 

This single cell a fertilized egg cell immediately divides and 
redivides. Soon protuberances, composed of groups of cells which 
are to become leaves, stem, and root, appear in the solid mass of 
cells. Then the seed coats harden, and the embryonic plant 


ceases to grow, awaiting favorable conditions for resuming growth. 
The whole structure has now become a seed. Its essential struc- 
ture is the embryo. Consider the oak tree again. We are accus- 
tomed to think that the plant begins its life at the time the seed 
germinates. This is not true. The embryo oak may have been 
resting for months or even years within the seed. The very begin- 
ning of the individual oak tree is the fertilized egg cell. 

Problem 2. How does the plant cell grow? 

We have learned that the plant body is composed of innumer- 
able units called cells. All many-celled living things grow by the 
multiplication and enlargement of their cells. Each cell con- 
sists of a minute mass of living substance, or protoplasm, enclosed 
by a more or less rigid, non-living wall. The living substance has 
two main parts: an outer region, the cytoplasm, which probing 
with an extremely fine needle proves to be of about the consistency 
of glycerin; and an inner slightly less fluid region, the nucleus, 
which seems to control the activity of the whole. Both are sur- 
rounded by delicate membranes. Throughout the cytoplasm are 
small, clear, watery areas called vacuoles, which contain cell sap 
(water plus other substances in solution). 

There are conspicuous differences between a young cell and 
an old cell. Near the growing point of any stem or root we can 
observe cells of different ages, and thus note the changes which 
they go through as they grow. 

Exercise 70. Growth of cells in a root tip. Examine prepared, specially 
stained, lengthwise sections of a root tip. Observe the youngest cells at the 
growing point, and cells of increasing age farther and farther back from the 
growing point. Compare the cells observed with Fig. 51. 

From the observations in this exercise we note the following very obvious 
changes in a growing cell: (1) enlargement, which is usually not equal in all 
directions; (2) increase in the size of the vacuoles, such that in an old cell the 
cytoplasm and nucleus merely line the inner surface of the cell wall; (3) in 
addition to these more obvious changes, there is often, in a growing cell, 
increase in thickness of the cell wall, and changes in the wall's structure and 
chemical composition. 

Exercise 71. Differentiation. Examine the prepared slides of Exercise 
70, and observe the different kinds of tissues distant from the growing point, 
that is, in the zone of differentiation. Here we see cells of many different sizes 



and shapes, with differences in the thickness of walls and markings on the 
walls. Clearly, these cells are fitted to carry on different functions. A point 
worthy of special note is that all the different kinds of cells back of the growing 
point have been derived from the same kind of cells those found at the grow- 
ing point. In the development of growing-point cells, some have taken one 
course, some another; some have become conducting elements, others storage 
elements, etc. This is a process called differentiation. 

One Larqe.; 



Cell uuall 





Cell Division 
Req ion 

Root Cap Cel Is 

Tqpical Cell Characteristics in Root Tip 

FIG. 51. The root tip at right, cut in lengthwise section. At left are cells 

(enlarged) taken from different zones of the root. Note that the cells become 

older the farther they are from the region of cell division. Furthermore, 

observe the changes in the cells as they mature. 


At the very growing point, in the region of active cell division, growth of a 
cell is soon followed by division of the cell. That is, a cell grows to a certain 
size, then divides into two similar cells, each resulting cell then doubling in size 
and dividing in two, and so on. The process of cell division is very complicated; 
it will be discussed in another chapter. Thus growth of this region involves 
both an increase in the size of cells and in the number of cells. 

Problem 3. What is the nature of seed germination? 

We have learned that the essential part of a seed is the young 
plant the embryo. The plant in the embryo stage of its develop- 
ment may remain alive for years. That is, some seeds have a very 
great longevity. Under proper conditions of moisture, tempera- 
ture, and supply of oxygen, the embryo starts to grow. This 
growth of the embryo, with the accompanying bursting of the seed 
coats, is called germination. Seed germination is essentially the 
resumption of embryo growth. Germination is completed when 
the young plant has developed far enough to lead an independent 
life, that is, does not derive nourishment from food stored within 
the seed. The food stored within the seed was obtained from 
the parent plant during the growing season. 

Exercise 72. Water and germination. Place a number (100 to 200) of 
dry seeds of corn, radish, wheat, or other different kinds of seeds in glass 
tumblers or wide-mouthed bottles with a substratum of cloth, sand, paper, or 
sawdust that is barely moistened; in a second series, use soaked seeds, placing 
them on substrata saturated with water; in a third series, cover the dry seeds 
with water. Keep all at same temperature. Record your conclusion. 

Water and germination. Water softens the seed coats and 
makes it possible for the young sprout to break through them more 
easily. Water also facilitates the entrance of oxygen into the seed, 
for when the seed coats are wet, oxygen will diffuse through them 
more readily than when they are dry, and too, carbon dioxide 
which is given off in the respiration of the living cells of the embryo 
can diffuse outward more easily. The secretion of digestive fluids, 
the digestion of stored foods, the movement of foods, in fact, all 
activities of the cells of the seed proceed only when they are well 
filled with water. Seeds which are old will not stand as much 
water as vigorous, fresh seeds. In handling old seeds, care must 
be taken to apply the water to them gradually and uniformly; 



variations in, the amount of water are injurious. Seeds will 
endure greater extremes of temperature when they are dry than 
when moist. Why? 

Temperature and germination. No less essential than water 
to seed germination is a proper temperature. The temperature 
which is the most favorable to germination is not the same for the 
different kinds of seeds. It is quite well known that cucumbers 
and melons require a higher temperature to germinate properly 
than wheat, barley, and certain other small cereals. The seeds 
of " cool-season crops " such as peas, lettuce, radish, and small 
cereals, will germinate readily at 50 to 60 F., but corn, pumpkin, 
cucumber, eggplant, and other " warm-season crops " require a 
temperature of 70 to 80 F. to give fairly rapid germination. 
It requires from 10 to 12 days for corn grains to germinate at a 
temperature of 49 F., whereas at 80 F. they will germinate in 
2 days. 

The lowest temperature at which seeds can germinate is called 
the minimum temperature; the temperature at which they germi- 
nate quickest, the optimum; and the highest temperature at which 
they can germinate is the maximum. These three temperatures 
for a number of different kinds of seeds are shown in the following 

Minimum F. 

Optimum F. 

Maximum F. 

Barley . . 




Clover (red) 




Cucumber . . . . 




































From 60 to 80 F. is satisfactory for the germination of most 
seeds of temperate regions. The temperature of the soil is referred 



to here, rather than the temperature of the air. When the soil 
is cold and moist, seeds should not be planted as deep as when it is 
warm and moderately dry. Why? 

Exercise 73. Temperature and germination. Use petri dishes, or other 
dishes with cover, as germinators, and in the bottom of each place several 

brush . 

FIG. 52. Three germinating stages in wheat. (From Robbins, in Botany 

of Crop Plants.) 

thicknesses of filter paper or moist cloth. In each, place 100 lettuce seeds. 
Subject these lots to different temperatures ranging from to 30 C. Keep a 
record of rate and percentage of germination. This exercise may be repeated 
using different kinds of seeds as desired. Draw conclusions. 

Suggested activity. Place a half pint of soaked peas in a thermos bottle. 
Insert a thermometer down into the peas, and pack cotton into the neck of the 
bottle around the thermometer. Observe the reading of the thermometer 
as sprouting of the peas proceeds. Explain any increase in temperature. 


Oxygen and germination. It must be kept in mind that all 
living cells of the seed must respire in order to maintain life, and 

that some oxygen is necessary in this process. In the resting stage 
the seed requires very small amounts of oxygen, but when germi- 
nation starts it demands a greater amount. That germinating 
seeds respire actively is shown by the large quantities of carbon 
dioxide they give off, and also by the heat liberated in the process. 
A mass of germinating seeds of barley, or other seeds which gerini- 

FIQ. 53. Germination of pumpkin (Big Tom) seeds, showing the pegs func- 
tioning in the removal of the coats. (After Crocker, Knight and Roberts, 
from Robbins, in Botany of Crop Plants.) 

nate rapidly, may actually become heated until they feel warm to 
the hand. Even though there is sufficient water and warmth, 
unless seeds are planted so that free oxygen can reach them, they 
will not germinate. If seeds are planted too deep in a heavy clay 
soil, or in a soil that is too wet, they are quite likely to have a poor 
supply of oxygen and to germinate slowly. Explain why seeds 
should not be stored in air-tight containers. 

Exercise 74. Oxygen and germination. Some seeds, those of rice for 
example, will germinate with a very small amount of oxygen, even with that 
which occurs in water. Place rice seeds in a beaker of ordinary tap water. 


In a second tumbler place some boiled water, allow to cool, place rice seeds in 
the bottom, and cover the surface of the water with a thin film of oil to prevent 
the absorption of oxygen by the water. Record results. Explain. 

Exercise 75. Process of germination. Observe stages in the germination 
of beans, peas, corn, squash, castor beans, or other seeds. Make comparisons. 
Examine each with regard to the cotyledons, the root, the hypocotyl, and the 
plumule. Which part first appears above the ground? Compare the growth of 
seedlings from which the cotyledons are removed soon after they come above 
ground with that of seedlings with the cotyledons intact. Test the cotyledons 
for starch. What is the function of the cotyledons? What is their fate? 

FIG. 54. Germinating seeds of mustard. The white cottony masses are 
root hairs on the primary roots. 

Write descriptions of the stages of germination, comparing the different 

The process of germination. The first stage in the process of 
germination of the seed is the absorption of water. The rate at 
which seeds absorb water from soil depends chiefly upon the 
water content of the soil, the compactness of the soil, its tempera- 
ture, and the character of the seed coats. A soil may be so dry 
that the seed does not absorb enough water to germinate, but re- 
mains in a dormant condition. Although a seed may absorb water 
very rapidly from a very wet soil, it will not necessarily grow so 
rapidly in such a soil. If a soil is excessively wet the oxygen supply 



in it is low, and oxygen is as essential to the growth of the embryo as 
is water. 

It is common practice to compact the soil over the seed after 
it is planted. This brings the moist particles close about the seeds 
and increases the points of contact between them. This object is 
attained by the use of the press wheel on planters. 

Seeds absorb water more rapidly from a warm soil than from a 
cold one. 

" Hard seeds." Although most seeds have coats which per- 
mit the ready intake of water, in some seeds, such as those of 

alfalfa, sweet clover, and other legumes, 
the coats may be almost impermeable 
to water. Such seeds are called 
" hard seeds." They will not grow 
readily, even when placed under per- 
fect conditions for germination. Hard 
seeds are not necessarily poor seeds. 
Some of them will germinate in several 
weeks; some will remain in the ground 
for an indefinite period without germi- 
nating. Unhulled sweet clover seed 
may often have as much as 85 per cent 
hard seed. Hulled sweet clover has 
a lower percentage of hard seed than 
the unhulled. Why? 

The permeability of seeds of le- 
gumes can be increased by " scarifying/' 
that is, passing them through a 
machine that scratches the surface. 
The ordinary alfalfa huller is effective 
for this purpose, as is shown by experiments; alfalfa grown under 
a variety of soil and climatic conditions had about 90 per cent of 
hard seeds if hulled by hand, and only about 20 per cent if hulled 
by machine. 

Digestion of stored food in seeds and its transfer. The second 
stage in the germination of the seed is the digestion of stored foods, 
and its transfer to the growing points of the young plant (embryo). 
Certain cells of the seed secrete digestive juices which act upon 

FIG. 55. Stages in the devel- 
opment of a cactus plant. At 
the left, the cotyledons of the 
seedling are just breaking 
loose from the seed coats. 



the stored foods, render them soluble, and make possible their 
movement to the growing points of the roots and stem. As the 
young plant grows, the stored food in the seed is used up. It is 

FIG. 56. Germinating seeds between the folds of Canton flannel. Use two 

dinner plates, one inverted over the other. (From Robbins and Egginton, 

in Colo. Agr. College Extension Bulletin.) 

important to keep in mind that all the early growth of the young 
plant is made wholly at the expense of this stored food. However, 
not all the reserve food enters into the plant substance of the 

FIG. 57. The seeds of bluegrass should be germinated on top of a blotter 

and kept in the light; cover the germinating dish with a plate of glass. (From 

Robbins and Egginton, in Colo. Agr. College Extension Bulletin.) 

seedling, but a portion of it is lost in respiration. It is not until 
the roots are established in the soil, and the leaves are green, that 



the plant is capable of making its own food and leading an inde- 
pendent life. 

Depth of planting seed. The depth at which a seed can be 
planted safely depends somewhat upon the amount of food stored 
within it. Many small seeds, such as those of tobacco and certain 
grasses, are planted on the soil surface; large ones may be planted 
more deeply. If a seed is planted so deeply that its reserve food 
supply is consumed before the plant reaches the light, the plant 
will die from starvation. Seeds should not be planted deeper than 
is necessary to insure a proper amount of moisture. 

Growth of embryo. The swelling of the seed caused by the. 
absorption of water, and the growth of the embryo, break open the 

FIG. 58. The dinner plate seed tester, (a) One hundred seeds are scattered 
on one-half of the blotter. The other half of the blotter is folded over the 
seeds, (b) Cover with another dinner plate, thus making a moist chamber, 
(c) The seeds have germinated, and the sprouts are ready to be counted. 
(From Robbins and Egginton, in Colo. Agr. College Extension Bulletin.) 

seed coat. The young root is usually the first part of the embryo 
to protrude. The cotyledons may remain in the soil or may be 
brought above the soil. The single cotyledon in all grains remains 
in the soil. The cotyledons of peas also remain underground. 
But in such seeds as bean and squash, the cotyledons are brought 
above ground and become the temporary leaves. Being exposed 
to the light, they may become green and aid in the food-making 
process. After a time, however, all the food that has been stored 
within them is absorbed by the growing seedling and they shortly 
wither and fall off. 

W&"^ ^ 


FIG. ay. Tne soil Hat or box tester, used in making individual ear tests of 
corn, (a) Number the squares on the cloth and ears to correspond; (b) place 
the kernels from individual ears on the squares; (c) cover the seeds with a 
second layer of Canton flannel, moisten, and cover with moist soil or sand; (d) 
at proper time remove the top cloth carefully, count and record the sprouts. 
(From Robbins and Egginton, in Colo. Agr. College Extension Bulletin.) 




FIG. 60. The rag doll tester, used in making individual ear tests of corn. 
(From Robbins and Egginton, in Colo. Agr. College Extension Bulletin.) 



Conditions affecting the vitality of seeds. It is a common 
observation that when a lot of seeds is planted under the most 
favorable conditions for germination a number of them fail to 
germinate. Some seeds are quick to germinate and form strong, 
vigorous seedlings. Others sprout but slowly, and the young plants 
are weak and sickly. The grower wants seeds which have a power 
to germinate readily and produce vigorous sprouts. In other 
words, he wants seeds of high vitality. 

Many conditions have an influence on the vitality of seeds; 
they are discussed in the following paragraphs: 

1. Vigor of the parent plant. Seeds from strong, vigorous 
parent plants usually have larger embryos and a greater amount 
of food reserve than those from weakly parent plants. In the 

FIG. 61. Making the purity test. (From Robbins and Egginton, in \joio. 
Agr. College Extension Bulletin.) 

selection of seed for planting, strong healthy mothers should be 

2. Conditions to which the seeds are exposed while developing. 
The vitality of the seed is influenced by the temperature of the air 
and the amount of moisture in the air at the time the seed is 
maturing. Most seed mature best under dry atmospheric condi- 
tions, and with moderately high temperature. Low temperatures 
early in the autumn may injure the partly mature seed. Corn, 
for example, sufl'ers from freezing before the grain is thoroughly 


dry. The tissue of the grain is broken down by the freezing 
of the water in it. If the grain becomes thoroughly dried, it will 
withstand very low temperatures. Corn containing 13 per cent 
moisture may be stored with safety in bins exposed to tempera- 
tures much below freezing. 

3. Maturity of the seed. Although seeds will often germinate 
when they are not fully ripe, the plants from such seeds are fre- 
quently weak. Lack of maturity or low vitality in corn is usually 
indicated by soft ears, by any discoloration of the grain, especially 

FIG. 62. Showing the method of dividing the original sample in order to 

obtain the proper amount for purity analysis. (From Robbins and Egginton, 

in Colo. Agr. College Extension Bulletin.) 

at the tips, and by blisters on the skin. Immature corn quickly 
loses its germinating power. 

4. Conditions under which seeds are stored. Seeds should be 
stored under conditions that are uniformly dry and cool. If the 
atmosphere is moist and warm, germination may be started; if 
it is, the respiration rate of the live cells is increased and the seed 
uses up a certain amount of its stored food. Its energy is thereby 
diminished. The seed may not have sufficient moisture and heat 
to germinate fully, but it will be kept in a greater state of activity 
than when in a completely dormant condition. Hence, its vitality 
is gradually being reduced. Moreover, in certain instances, seeds 
in bulk stored under moist, warm conditions may " heat " to such 
an extent that the embryos are actually killed by the high tem- 


5. Age of seeds. It is well known that seeds gradually lose 
their vitality as they grow older. The rate at which they lose their 
vitality depends upon the kind of seed and upon the conditions 
of storage. Seeds containing oil, such as corn and flax, lose their 
vitality much quicker than starch-bearing seeds, such as those of 
legumes. The seeds of legumes are noted for their great longevity. 
Some have been known to retain their vitality for 150 to 200 years. 
We hear the claim that wheat grains taken from the ancient tombs 
of Egypt will germinate. What is your opinion of this? 

FIG. 63. The purity test. With a knife and with the aid of a tripod lens 

the sample of seed is separated into three piles; (a) pure seed, (b) weeds 

and other foreign seeds, and (c) inert matter. (From Robbins and Egginton, 

in Colo. Agr. College Extension Bulletin.) 

Delay in the germination of seeds. The seeds of many plants 
have a rest or dormant period. That is, they will germinate better 
after a period of rest than they will when first mature. This 
dormancy is more common among wild plants than among domes- 
ticated ones. For example, wild oat seeds experience a delay in 
their germination, seldom germinating the year they are formed. 
The seeds of a number of weeds will lie in the ground for years in a 
dormant state. It has been shown that some seeds are still viable 
after 30 years 7 burial in the soil. Among such are the seeds of 



pigweed, black mustard, shepherd's purse, common dock, green 
foxtail, and evening primrose. 

There is an old saying that one year of seeds means seven years 
of weeds. A crop of seeds is borne ; some of them may germinate 
immediately if the conditions are favorable; others may remain 
dormant for a year or two, and still others may remain dormant 
for five or six or more years. In cultivation, the seeds may be 
buried to such a depth that they do not get enough oxygen to 

germinate. Consequently, 
they lie dormant in the 
soil. Later, perchance, 
they may be turned to 
the surface in plowing and 
brought under conditions 
favorable to their germi- 

The delay in the ger- 
mination of seeds may be 
due to several causes. 
Probably the most com- 
mon cause is an impervious 
seed coat which prevents 
or retards the absorption 
of water. This topic was 
discussed on page 126. 
Another cause of dormancy 
is the inability of the 
embryo to break the seed 
coat. This is true of the 
common pigweed seed. 
As the seed lies in the ground, freezing and thawing and the 
action of soil organisms gradually soften the coats and make 
germination possible. Still another cause of seed dormancy is 
the inability of the embryo to germinate until it has gone through 
a series of changes known as " after-ripening. " This process 
may be hastened in some instances by exposure of the seeds to 
low temperatures. 

" Hard seeds " (see page 126), whose coats are impervious to 

FIG. 64. A common commercial type of 
seed germinator. The cloths or blotters 
holding the seeds are placed on the sliding 
trays. The germinator is heated by an 
electric plate and the temperature is con- 
trolled by a thermostat. 


water, may be hastened in their germination by scratching the 
surface. The delay in the germination of olive seeds, which have 
a stony covering, may be overcome in part by soaking them in 
warm water, soaking in alkaline or acid solutions, or clipping the 
ends. Germination of some seeds may be hastened by soaking 
them in water before planting. For example, asparagus seed 
soaked a period of three to five days at a temperature of 75 to 
85 F. will germinate more quickly than unsoaked seeds. The 
seeds of beets and lettuce have their germination hastened by 
soaking for a period of six hours in water. 

Such hard-coated seeds as those of the peach, cherry, and 
walnut are often stratified in the early winter and permitted to 
freeze and thaw in order to break the seed coats. In stratifying 
seeds, alternate layers of sand and seeds are put in a box. They 
are then placed in a well-drained place and allowed to freeze. 
In mild climates where winter freezes seldom occur, the germina- 
tion of many seeds is improved by stratifying them in a moist 
place; the moisture and the temperature fluctuations are probably 

Suggested activity. Make rag doll seed testers according to methods 
given in farm bulletins and test corn grains from different ears obtained from 
various sources. What is the practical value of seed testing? 


1. In what states has the vegetable and flower seed industry been developed 
extensively? Why? 

2. Why have seeds of legumes, as a class, relatively great longevity? 

3. What is the probable explanation of the belief that " wheat changes to 

4. Do you believe that all farmers should test, or have tested, their seed 
before planting? Give reasons. 

Problem 4. How do stems grow in length? 

In order to understand the manner in which stems grow in 
length, it will be necessary for us to be familiar with their external 
characters, and with the structure of buds. 

Exercise 76. Twig characteristics. Examine a leafy twig of some woody 
plant, the cottonwood, for example. Observe that it is divided into sections 



(internodes), and that at the enlarged joints (nodes) the leaves arise. Buds 
develop in the axils of the leaves, and also at the tip of the branch. The bud 

at the tip of the twig is called 
the terminal bud. Those along 
the side of the stem at regular 
intervals are lateral buds. The 
terminal bud develops the 
following spring into a branch, 
which, in turn, bears leaves. 
A lateral bud may be a leaf 
bud or a flower bud. If a 
leaf bud, it will develop into 
a side branch bearing leaves; 
if a flower bud, it will bear 

Exercise 77. The structure 
of buds. Make cross- and length- 
wise sections of a large terminal 
bud. Note that it consists of 
a short axis, bearing small 
leaves. The outer leaves of the 
bud are called scales; they pro- 

latent bud*- 

\terminal bud- scar 

terminal leaf-bud 

[ flower-bud* 


/ la 'feral leaf-buds 

. flower-bud-scars 

one year 
old branch \ 

FIG. 65. Cotton wood twig two years 

old. (After Longyear from Robbins, 

in Botany of Crop Plants.) 

FIG. 66. Section of stem 
showing a shedding leaf; also 
bark, wood, and pith are seen 
in cross- and longitudinal sec- 
tions. (After Longyear, from 
Robbins, in Botany of Crop 



tect the soft, tender tissue within from drying out and from mechanical 
injury. The inner leaves are undeveloped foliage leaves. 

Examine lengthwise sections of a terminal bud, made thin enough so that 
the structures may be studied with a compound microscope. At the very tip 
of the stem is a region made up of cells which are capable of division; back of 
this is a region in which the cells are rapidly elongating; then, farther back, is a 
region in which various stem tissues are differentiating; and still farther back 
is the mature part of the stem. It will be observed that within this bud there 
are very short internodes, and that the leaves come off at regular intervals, 
following identically the same arrangement as in the adult twig. 

Exercise 78. Examine preserved specimens of twigs showing buds in the 
process of swelling and opening in early spring. What is happening to the 
young shoot which was hidden and protected by the scales of the bud during 

FIG. 67. Growth as shown in opening buds of hickory. Left, opening bud; 
center, section of opening bud; right, growing shoot from a recently opened bud. 

the winter? Explain why it is possible for a shoot to make such rapid develop- 
ment in early spring. 

It is apparent from the above studies that a bud is simply an 
undeveloped stem. A bud is a very short, young stem in which 
the internodes are exceedingly short. The growing point of 
a stem, then, consists of a number of very much shortened inter- 
nodes; growth in length of the shoot consists in the lengthen- 
ing of these internodes by increase in the number and the size of 
cells that compose internode tissue. When a twig has made its 
year's growth, the internodes do not lengthen during subsequent 
years. Increase in length of that shoot is due to the addition of 



other " joints " at the end. The fixed length of old internodes is 
well proved by the common observation that nails driven into the 
trunk of a tree, or a branch, are not elevated above the ground as 
the tree grows. If, when you were a young boy, you carved your 
initials deep in the bark of the old tree that grew by the swimming 
hole, those initials, although probably partly obliterated by the 
growth of bark, ate today at the same distance above the ground 
as they were the day you carved them. A common impression 
prevails that, in pruning, the branches of a young tree should 

FIG. 68. The beauty of our landscapes is being marred by tree-butchery, 
such as is shown here. 

be started low to the ground, so that they will be at about the 
proper elevation above the ground when the tree reaches maturity. 
The errroneous supposition here is that the limbs are raised by the 
growth of the tree. 

Problem 5. How do stems grow in diameter? 

Exercise 79. Structure of the woody stem. With a safety razor blade 
cut thin cross-sections of a one-year old woody stem, such as a twig of the 
cottonwood, box elder, cherry, or apple. Note the three principal regions : the 



bark, the wood, and the pith. The bark can be separated from the wood. 
It separates from the wood along a region known as the cambium. The 
cambium is composed of thin-walled, tender cells, capable of rapid division 
and growth. The cambium is the growing layer of the stem. (See p. 64). 

The bark is covered with a corky layer which successfully pre- 
vents the rapid loss of water from the stem. Beneath the corky 
layer of the bark are several layers of cells containing chlorophyll, 
and hence capable of manufacturing sugar. The inner part of the 
bark is the phloem. The phloem is that portion of the stem which 

Conducting Tube 


-Heart Wood 

~-~ Rays 

Spring Wood 
Summer Wood 

Annual Ring 

FIG. 69. Portion of a four-year-old stem of the pine, shown in transverse, 
radial, and tangential views. (Redrawn from Strasburger.) 

is largely concerned in the conduction downward of foods manu- 
factured in the leaves (probably mineral substances and foods 
upward, also). Large tubes, known as sieve tubes, in the phloem 
are the conducting elements. In addition to cork, chlorophyll- 
bearing tissue, and phloem or food-conducting tissue, the bark 
may have fibers and other cells which give strength. 

The wood of the stem is made up chiefly of large conducting 
tubes or vessels, fibers, and storage cells. It is in the vessels that 
water (and probably salts and foods, also) is carried. The fibers 


?ive strength to the stem. The storage cells store water and 
foods, and may also conduct these substances short distances. 

The pith of the stem consists of a group of large, thin-walled 
cells which store food to some extent. The amount of pith in 
stems varies greatly. 

Radiating from the pith, and extending through the wood and 
the phloem part of the bark, are rows of cells which constitute the 

FIG. 70. Cross-section of six-year-old woody stem. Note the dark-colored 
heartwood, light-colored sapwood, bark (dotted) and vascular rays. 

vascular rays. Water, salts, and foods are carried radially in these 
ray cells; they also serve as places of food storage. If a cross- 
section of a twig is treated with iodine, starch, which is stained 
blue, is seen to occur chiefly in the vascular rays and in the outei 
cells of the pith. 

Exercise 80. Structure of woody stem two years old. Cut sections as 
in Exercise 79, but of two-year-old twigs. Compare with the one-year-olc 



stem. With a hand lens determine the number of rings of growth of wood. 
Note the " pores/' the vessels as seen in cross-section. (Fig. 31). 

In stems of the type to which our common orchard trees belong, 
there is a continuous 
cambium layer be- 
tween the bark and 
the wood. The cam- 
bium cells divide and 
redivide, adding to 
the bark cells on the 
outside and to the 
wood cells on the in- 
side. Hence, by a di- 
vision of cambium 
cells, new phloem is 
laid down on the inside 
of old phloem, and 
new wood is laid down 
on the outside of the 
old wood. A layer of 
phloem and a layer of 
wood are formed each 
year. The phloem 
rings are less distinct 
than those of the wood, 
and as the stem grows 
older the older phloem 
may peel off with other 
bark tissue. 

Annual rings of 
growth. An annual 
ring, as generally un- 
derstood, is one year's 
growth of wood. The 
ring varies in width, 
depending upon the 
time in the life of the plant it was formed, and upon seasonal 
and climatic conditions. Furthermore, it is known that some 

FIG. 71. Cross-section of a portion of pine wood. 
One complete annual ring (center), and parts of 
two other annual rings (above and below) are 
shown. The narrow, dark part of the annual 
ring is " summer wood/' the broad, light part, 
"spring wood." 



' ^'-' ; * ' 


I m 

trees grow rapidly, producing wide annual rings, whereas it is 
a specific character of others to grow slowly, i.e., produce nar- 
row rings. The amount of carbohydrates supplied by the leaves 
and the water supply are two chief factors determining the width 
of rings. 

There is usually a marked difference in the wood formed in the 
spring and early summer and that produced in late summer and 

fall. In early or so-called 
spring wood, conducting 
tubes are large and quite 
numerous; in late or 
summer wood, conducting 
tubes are smaller and fewer, 
and wood fibers are rela- 
tively more abundant. 
Hence, summer wood has 
more strength than spring 
wood. The summer wood 
of one year (say 1925) is 
adjacent to the spring 
wood of the following year 

Soft wood is usually 
one from a tree which 
grows rapidly. The con- 
ducting tubes are rather 
small and uniform in size 
and evenly distributed 
throughout the year's 
growth. Hard wood is 
usually a comparatively 
slow-growing wood. The 
conducting tubes of the 

spring and early summer are large and numerous, but the autumn 
wood is solid as a consequence of the greater abundance of strength- 
ening elements. 

. . 


E" '"''TV*'- ' 7v,, ;;,:.,,,, 
: '**ai!iiis?* s 

FIG. 72. A portion of pine wood cut length- 
wise, showing the conspicuous, circular bor- 
dered pits in the walls of tracheids. 

Exercise 81. Determining the age of trees, 
or log, attempt to determine the age of a tree. 

Using a freshly cut stump 
If the rings of growth are 


counted on a stump three feet high, does this represent the true age of the tree? 
Why? Can we always rely absolutely upon the number of rings in determining 
the age of a tree? Why? 

Cork. Usually when we speak about the growth in diameter 
of a woody stem, we refer to the annual rings of wood, that is, of 
tissue inside the cambium. The bark also develops annual layers 
but they are much thinner than those in the wood and generally 
are broken and split off. Also, woody plants develop a cambium, 
known as cork cambium, which usually originates in the cortex. 
This cambium forms cork to the outside and cortex to the inside. 
In some plants, notably the cork oak of commerce, the layers of 
cork, formed year after year, adhere to the tree, and we observe 
them as definite annual layers of growth. Cork cells have walls 
which are impregnated with a fatty substance known as suberin, 
which is impervious to water. 

Summarizing, the growth in diameter of a woody stem is due 
to the activity of two cambiums: (1) the vascular cambium which 
lies between the wood and bark; this adds new rings of growth of 
wood to the outside of the old wood, arid new bark tissue to the 
inside of the old bark ; (2) the cork cambium, situated in the bark ; 
this adds layers of cork to the inside of old cork, and cortex tissue 
to the outside of old cortex. In most woody plants there is a 
gradual peeling off of the bark, which includes tissues arising from 
both cambiums. 

Suggested activities, (a) Find out by inquiry and from books on horticulture 
how fruit trees are grafted, and prepare a report to be read to the class. Ex- 
plain why it is necessary to bring the cambiums of stock and scion into contact. 
Include in your report a description of " budding " as done by fruit-growers. 

Problem 6. How do roots grow? 

In Problem 2, Exercises 70 and 71, we examined young roots 
and found that the growth in length of a root is near the tip. This 
may be ascertained by the following simple experiment. 

Exercise 82. Method of growth in length of a root. Germinate horse 
beans or lima beans on moist cloth or filter paper in a covered dish. When the 
first root is 1 or 2 inches long, carefully mark with lines of India ink, 1 mm. 
apart, beginning at the tip and extending backwards 2 or 3 cm. After 24 


hours, observe. Which marks are the farthest apart? Draw conclusions as to 
the regions of growth. 

The growing point, which includes cells capable of division and 
growth, is some distance from the root tip, being covered and pro- 
tected by a root cap. Immediately back of the growing point, the- 
cells are elongating, and still farther back the cells are differentiat- 
ing. As a matter of fact, the growth in length of a root is confined 
to the growing point and region of elongation, these two regions 
together usually being not more than \ to \ inch long. A root is 
not pushed through the ground by growth of cells far removed 
from the tip. Rather, by the addition of new cells immediately 
behind the protective root cap, and their elongation, the root tip 
finds its way between the particles of soil. There are no joints in 
the root as there are in the stem. Why is it practically impossible 
for a root to pursue a straight course through the soil? 

The growth in diameter of the roots of perennial plants is simi- 
lar to that in the stems. An old root of our common trees and 
shrubs has annual rings, and very much the same structure and 
appearance as an old stem. Can you cite an example of the lifting 
power of roots? 

Problem 7. How do leaves grow? 

We know that leaves grow very rapidly in the spring. After a 
few warm days, the entire tree appears green, and in two or three 
weeks leaves have attained their maximum size for the season. 
It must be that leaves are fairly well formed in the bud. This fact 
is well demonstrated by the following exercise. 

Exercise 83. The growth of leaves. Remove the scales from winter leaf 
buds of several different kinds of deciduous plants and carefully dissect out 
the young foliage leaves, observing whether they are rolled, folded, or plaited. 
Spread these young leaves out flat, examine with binoculars, and observe that 
even in the bud the leaves have veins, and very much the form they will have 
when fully grown. 

From these observations we are led to conclude that the leaves 
of our common temperate-climate deciduous plants are formed the 
season before their expansion. When the bud breaks open in the 



spring, the leaves grow very rapidly, attaining full size within two 
or three weeks. 

We have learned that stems and roots grow in length chiefly at 
or very near the tip. There is a very unequal rate of growth in 
different parts of these organs. Not so 
with leaves, as is shown by the following 

Exercise 84. The growth of leaves. With a 
leaf-marker (rubber stamp marked into millimeter 
squares) stamp a young leaf, y% to 1 inch in width. 
After the leaf has attained full size, observe the size 
and shape of the squares. If throughout the leaf the 
squares have maintained their shape, it is an indi- 
cation that growth has been at an equal rate 
throughout all portions of the leaf. As a matter of 
fact, growth of the leaf after it breaks from the 
bud is simple enlargement of cells already formed; 
additional cells are not developed. 

Thus far we have not accounted for the 
growth and development of leaves in the 
bud. From Exercise 77, we learned that 
the stem growing point consists of a very 
short axis with nodes and extremely short 
internodes. The nodes are the places on 
the stem where the leaves arise. In the 
lengthwise sections studied in the above 
exercise, we observed slight protuberances 
near the growing point, which consisted of 
groups of cells, each destined to become a leaf. Each group of 
cells finally enlarges and takes on the form of a leaf, which rests 
in the bud stage until the spring of the following year. 

FIG. 73. A single fern 
leaf unrolling. 

Problem 8. How do seeds and fruits grow? 

In the^ discussion of Problem 1, it was pointed out that seeds 
develop within a certain structure of the flower known as the 
pistil, and that long before the flower opens there develops within 
the pistil one or more small masses of tissue, each of which is de&- 


tined to become a seed. These masses of tissue are called ovules. 
The ovule is a small spherical or egg-shaped structure in the ovary 
of the pistil. It is attached to the ovary by a short stalk, which 
becomes the stalk of the seed. The mature ovule, just before fer- 
tilization, consists of a central mass of tissue, surrounded by one 
or two coats which become the protective coats about the mature 
seed. These fit closely about the ovule, except at one point, where 
there is a very small opening, the micropyle. See p. 154. 

Within the central mass of tissue is the embryo sac, the struc- 
ture in which the embryo or young plant develops. The mature 
embryo sac commonly has eight nuclei, one of which, after fertili- 
zation, develops into the embryo plant; two others unite with a 
second nucleus from the pollen tube, and the resulting body devel- 
ops into endosperm, which is a food supply surrounding the 
embryo. The remaining five nuclei usually soon disappear, being 
absorbed or disintegrating. 

Pollen grains play a part in the formation of fruit. They are a 
product of the anthers. At maturity, the anthers split open and 
the pollen grains are distributed. The pollen grains of plants vary 
widely in form, size, color, and particularly in surface markings. 
The wall of the grain usually consists of two coats, an outer thick 
one and an inner thin one. The wall encloses a mass of proto- 
plasm, the essential parts of which are three nuclei. One of these, 
the tube nucleus, plays a part in the growth of the pollen tube; 
the other two, sperm nuclei, fertilize certain nuclei in the ovule. 

Ferilization. The pollen grain is usually brought to the stigma 
by wind or insects. It absorbs water and nutrient materials from 
the surface of the stigma, and grows by sending out a tube, known 
as the pollen tube. The pollen tube grows downward through the 
stigma and style and finally reaches the ovule. It goes through 
the micropyle and penetrates the ovule tissue. After the dissolv- 
ing of the wall at the tip of the pollen tube, the three nuclei are 
discharged into the embryo sac. The tube nucleus is absorbed. 
One sperm nucleus unites with the egg or female nucleus to form 
the fertilized egg. Thus, this nuclear mass contains determiners 
for characters from the plant furnishing the pollen (paternal char- 
acters) and also those from the plant fertilized (maternal charac- 
ters). The union of the sperm nucleus of the pollen tube with the 


egg nucleus of the embryo sac is fertilization. The fertilized egg 
nucleus now develops into a young plant (embryo). 

In cereals and lilies and a number of other plants, so-called 
double fertilization has been observed. One sperm nucleus has 
been accounted for as uniting with the embryo nucleus. The other 
unites with the two so-called polar nuclei of the embryo sac. The 
body resulting from this union also carries determiners for both 
maternal and paternal characters. It develops into the endo- 
sperm of the seed. 

Immediately following fertilization, there is a series of changes 
not only in the ovule, resulting in a seed, but in the ovary wall as 
well. Normally, if the egg nucleus is not fertilized the ovule does 
not develop, but withers and dies. 

Just one pollen tube penetrates the embryo sac to bring about 
fertilization. Many pollen tubes, even hundreds, may grow down 
the style, although comparatively few may function. Those which 
do not, wither and die. We may be sure that every ovule that 
develops into a seed has been visited by at least one pollen tube, 
and that only one pollen tube has functioned there. 

Summarizing : The seed develops from the ovule in the ovary, 
but ordinarily only after fertilization. After fertilization, the 
embryo or young plant develops from the egg nucleus, the endo- 
sperm develops from other nuclei in the embryo sac, the ovule 
coats harden to form the seed coats, certain tissues disintegrate, 
and the whole resulting structure we call a seed. The fruit is the 
matured ovary, with its seeds, and any other part of the flower 
which may be closely associated with it. The fruit contains ..the 
seed or seeds. For example, the entire bean pod is a fruit; the 
beans within are the seeds. It is often difficult to realize that a 
large fleshy fruit, such as a tomato, is derived from the ovary. 
The walls and partitions of the ovary enlarge greatly to form the 
mature fruit. But, throughout all the changes which occur during 
the development of the tomato fruit from a small structure much 
less than \ inch in diameter to the large tomato, there is very little 
increase in the number of cells; rather, simple enlargement of cells 
already formed, coupled with chemical and physical changes which 
affect texture, color, flavor, and edibility. 



1. What is the force which pushes young roots through the soil? 

2. How do cells of the young root change to vessels and tubes as the root 
grows older? 

3. Why is it that moist soil should be packed about seeds that are planted? 

4. Explain why plants in clay soil should never be cultivated when the soil 
is wet. 

5. Explain why corn is cultivated by digging the soil deeply at first, 
whereas shallow cultivation is used around older plants. 

6. Why should seeds never be planted in soil which is either very dry or 
very wet? 

7. Give two reasons why oats should be planted earlier in the season than 

8. Explain why only the tips of asparagus shoots are tender. 

9. Why is the bark of tree trunks usually ridged? 

10. Why is the surface of twigs of a tree more smooth than that of the 

11. If it requires 30 feet of rope to make a swing by tying the two ends to a 
branch of a tree, what length of rope will be required for a swing attached to 
the same branch 30 years later, the tree having increased in height 20 feet 
during the period? 

12. In what two ways can you tell the age of a twig? 

13. Why is the bark of a tree thinner than the wood? 

14. Explain the appearance of the grain of lumber. 

15. Explain why it is possible for rabbits to kill young trees by gnawing the 
bark from a ring around the base. 


Reproduction is one of the fundamental characteristics of life. 

Reproduction is race preservation. It is a process which occurs not 
only in animals, but also in all plants in trees and shrubs and 
herbs, in toadstools and ferns, bacteria and seaweeds in short, 
in every kind of plant existing on the surface of the earth. It is a 
process which, in the broadest sense, involves the production of 
new individuals, by any method whatsoever. 

An individual plant is designed to function not only for itself, 
but also for the race to which it belongs, to sacrifice all or a part 
of itself in propagating the species. 

All life comes from pre-existing life. The new organism is 
nothing more or less than a piece of living material separated from 
its parents. Living things as we know them originate only by 
reproduction. Even the tiniest germ visible with the most power- 
ful microscope must have ancestors. Fossil remains bear witness 
that millions of years ago, in the waters of ancient seas, life first 
appeared. From that day to this there seems to have been an 
unbroken continuity in the chain of living things. 

There are two general methods of reproduction of plants, 
namely, asexual reproduction and sexual reproduction. When 
you divide dahlia roots, or start " slips " and cuttings from roses 
and other woody plants, you are reproducing these plants asexu- 
ally, that is, without sex. Many of the primitive plants, such as 
the bacteria, reproduce by asexual means alone. With them, 
sexual reproduction is unknown. Sexual reproduction in plants 
involves the union of parts of two parents the egg of the female 
plant and the sperm of the male plant to form a new individual. 
All eggs and sperms are cells minute units of living substance or 
protoplasm. The cell resulting from the union of the egg and 
sperm grows into a mature plant. The cell is the unit of reproduc- 






V- Pistil 

Problem 1. How do flowering plants reproduce? 

In the higher plants, including cultivated plants of all kinds, 
the flower is the organ of sexual reproduction. It is in the flower 
that the seed is developed. Primarily the flowers are organs of 
seed production. Many flowering plants can be multiplied by 
means of vegetative organs, such as stems, roots, and sometimes 

leaves, but the princi- 
pal way in which they 
multiply is by means 
of seeds, which are a 
product of the flower. 

Flowers are exceed- 
ingly various. There 
are flowers so small 
that their organs are 
scarcely visible to the 
unaided eye; such are 
the flowers of the duck- 
weed or duckmeat 
(Lemna), which aro 
free, floating plants 
common in ponds 
throughout the world. 
Then there are the 
flowers of a tropical 
plant (Rafflesia), grow- 
ing on the floor of dark 
forests, which are as 
much as a yard in 
diameter. A great host 

of flowers like those of grasses, cotton-woods, and oaks are adapted 
to wind-pollination, whereas others, like those of orchids, snap- 
dragons, and mints, are so peculiarly constructed that the pollen 
is distributed only by certain insects whose bodily form enables 
them to enter the flower. The flowers of grasses, cottonwoods, 
birches, and many other wind-pollinated plants have no showy 
bright-colored parts; insect-pollinated flowers, on the other hand, 


: -> Receptacle 


FIG. 74. Diagram of a flower from which all 
but one of each whorl of flower parts have been 
removed. (Modified after Hall. From Hoi- 
man and Robbing, in a Textbook of General 



are usually gaudy and conspicuous. We might go on enumerat- 
ing the great number of variations in the size, color, structure, 
and form of flowers, but lack of space prohibits. 

Let us now familiarize ourselves with the structure of some 
typical flower one which has all parts present. 

Exercise 85. The parts of a flower. Examine the flowers of some plant, 
such as cherry, sweet pea, radish, or lily. The following principal parts will 
be observed: 

1. The sepals, green 
structures; taken together 
they form the calyx. The 
calyx covers the other 
flower parts in the bud. 

2. The petals, showy, 
colored structures; taken 
together they form the 

3. The stamens, slen- 
der structures, each with 
a thread-like stalk or fila- 
ment at the end of which 
is an anther. The anther 
produces a yellow powder 
called pollen. 

4. The pistil, the cen- 
tral structure of the flower. 
The parts of the pistil 
are the ovary, the swollen 

FIG. 75. A lengthwise section of an apricot 
flower bud, long before it is ready to open. 
Observe the immature ovary in the center, the 
stamens, the petals and sepals, and the over- 
lapping bud scales. (Photograph furnished by 
Division of Pomology, California College of 

base which contains the 
ovules, that is, the struc- 
tures which later develop 
into seeds; the stigma, the 
topmost part of the pistil 
which steals the pollen 
from insects or wind or 

other agency which trans- 
ports it; and the style, the slender part of the pistil which connects the 
stigma with the ovary. 

Within recent years scientists have found that all flowering 
plants bear microscopic sexual plants as parasites in their flowers. 
Just so is the human embryo a parasite upon its mother. The 
yellow pollen grains are nothing more or less than male plants; 



and hidden within the young seeds (ovules) are the parasitic female 
plants. The germ cells, that is, the eggs and sperms, are not 
produced directly by the flowers. Instead, flowers develop these 
email sexual plants which in turn bear the eggs and sperms. 

Within each anther 
there are developed a 
number of spores, a 
peculiar type of cell 
which, unlike eggs and 
sperms, is capable of 
growing into a plant 
without entering into 
the mysterious process 
of fertilization. Each 
of the spores in the 
anther grows into a 
minute male plant, a 
pollen grain. When 
the anther dries up 
and splits open, pow- 
dery masses of yellow 
male plants are carried 
by insects or wind to 
the pistils, inside of 
which the female plants 
are waiting. 

Exercise 86. The pollen 
grain. With the compound 
microscope examine the 
pollen grains of some flow- 
ering plant. In specially 
stained pollen grains will 
be seen the protective coat 
enclosing two cells. The 
nuclei of these cells are 
visible. Thus, it is seen 

that the pollen grain is not a single cell, but in reality a small sexual plant 

consisting of but two cells. 

Exercise 87. Germination of pollen grains. The pollen grains of many 

plants will germinate in a 10 per cent solution of cane sugar. Prepare hanging 

FIG. 76. An apricot flower bud just before 
opening. The ovary, covered with hair, is 
seen in the center, and above are the anthers. 
(Photograph furnished by Division of Pomol- 
ogy, California College of Agriculture.) 


drop cultures of a number of different kinds of pollen in the above solution. 
Germination of the pollen grain, like that of the seed, is resumption of growth. 
Under favorable conditions the two-celled male plant (pollen grain) germinates, 
germination consisting of the growth of a long tube the pollen tube. One of 
the cells divides to form two sperms or male elements. These may be seen in 
properly stained material, usually occupying a position near the end of the 
tube. In the mature pollen tube may thus be seen three nuclei, a so-called 
tube-nucleus and two sperm nuclei. These nuclei are accompanied by some 

The pistil is the young seed pod. Inside of each potential 
seed, which in the early stages is called the ovule, there is a single 
female plant. This is a minute, swollen bag and is called the 
embryo sac. At the end of the female plant or embryo sac, near- 
est an opening which is always left in the seed coats, there lies the 
cell which is to be fertilized. This is 
the egg cell, the female element. Such a 
cell, wherever found, whose sole function 
is union with a male cell, is called an egg. 

Exercise 88. Structure of the ovule. Split 
open the pistil of a flower and observe the one or 
more ovules. The internal structure of these can 
be studied only by appropriate microscopic sec- piQ ?7 __ Cross . section of 
tions. The central part of the ovule consists of ft matufe anther gh 
a mass of tissue called the nucellus Embedded ^ f(mr chambers 

within it is the embryo sac, the female plant. ... n 

" . . : * A . r containing pollen grams. 

Entirely surrounding the nucellus, excepting for 

one small opening, the micropyle, is a protective 

layer, consisting of one or two coats, the seed coats. Within the embryo sac 
are a number of cells, one of which, the egg cell, after union with a sperm 
from the pollen grain, grows into a new plant. 

Fertilization. When the embryo sac or female plant in the 
ovule is mature, the stigma is usually moist and somewhat sticky 
and conditions upon its surface are such as to cause the young male 
plant, the pollen grain, to resume its growth. In its growth it 
becomes, as we have seen, a microscopic, hair-like tube, the pollen 
tube. This tube grows down inside the pistil, through the micro- 
pyle, and into the female plant. The end of the tube bursts, 
emptying into the female plant the two sperm cells of the male. 
A sperm slowly dissolves itself in the egg. The two become one. 
This union of a cell from the male plant with a cell from the female 

The qamete plant staqe 
(qametophL|te) beqins vMh 
the formation of microspore 
and meqaspore and end& 
with the fertilization of 
the eqq; 

The spore plant staqe 
(sporopht|fe) bcqins with 
fertilization in a f \ower and 
ends in tne forminq of 
>pores in a flower of 
the off-sprinq 




FIG. 78. Life cycle of a seed plant with an enclosed ovule (Angiosperm). 
The seed (s) germinates and develops into the mature spore plant (s.p.). 
In the flower of the plant the sex organs, pistil (1) and the stamens (2) appear. 
The flower produces two different kinds of asexual spores. Within the ovule 
(3) the megaspore (4) develops. This megaspore germinates and goes through 
the stages, 5, 6, 7, in developing into the female gamete plant (f.g.) which 
produces an egg (8) and a fusion nucleus (9). The pollen grains germinate 
and develop (11, 12) into the male gamete plant (m.g.) with a pollen tube (p) 
containing a tube nucleus (n) and two sperm cells (a, b). In developing, the 
pollen tube grows down through the style of the pistil and around the ovule 
to the micropyle where it enters the ovule. The end of the pollen tube enters 
the female gamete plant where its wall dissolves, setting free the cells, a and b. 
The union of a male gamete with the fusion nucleus results in the development 
of the endosperm (stored food) of the seed. The union of b with the egg (fer- 
tilization) results in the forming of the embryo spore plant in the seed. The 
seed coats result from the development of the outer coats (integument) of the 
ovule. The seed usually goes through a dormant period before germinating. 




plant is fertilization. The process of fertilization, wherever it 
occurs in the plant and animal kingdoms, is really the same, in 
that it is the union of two masses of 
living material, a sperm and an egg. 

The fertilized egg immediately di- . 
vides and redivides. Soon, it changes 
from a shapeless mass to one showing 
the beginnings of leaves, stem, and 
root. Then the seed coats harden and 
the embryonic plant ceases to grow, 
awaiting favorable conditions for re- B 
suming growth. The whole structure 
has now become a seed. Its essential 
structure is the embryo the result 
of fertilization of an egg by a sperm. 
The embryo is a new plant, borne for s-< 
a while by the mother plant. Inas- 
much as one mother plant may produce 
thousands of seeds, there is a great 
multiplication of individuals. This is 
reproduction. j} 

Parthenogenesis. Normally, as 
stated, the egg or female gamete will 
not start on the train of changes which 
result in the embryo plant unless a 
sperm or male gamete fuses with it. 
Rarely, however, the embryo develops 
from an unfertilized egg nucleus. This 
phenomenon is called parthenogenesis. 
It is a rare occurrence among plants. 
The phenomenon has been observed in 
the dandelion, in hawkweeds, meadow 
rue, and several other groups of flower- 
ing plants. Also it has been observed 
in certain of the lower plants, chiefly 

Parthenocarpy. As a general rule, 
lack of fertilization of the ovules is 

FIG. 79. The earliest stages 
in the life of a plant. This 
shows how a plant starts out 
in life. The sperm nucleus 
moves into position beside 
the egg nucleus, as shown in 
A and B. The two divide 
side by side as shown in C 
and D so that there are two 
resulting groups of six chro- 
mosomes, instead of four 
groups of three. Each of the 
two groups then forms a sin- 
gle nucleus, and a cell wall 
forms between them. (From 
Robbins and Pearson, in Sex 
in the Plant World.) 



followed by the shedding of the blossoms; the fruit fails to 
develop completely if a good number of the ovules are not fer- 
tilized. However, development of the ovary does sometimes occur 
although fertilization fails. Such an unusual development is 
called parthenocarpy. With certain sorts of both apples and 
pears, fruits have been developed without fertilization. Of course, 
parthenocarpic fruit is seedless. There are among cultivated 
plants many which bear seedless fruit. Seedless tomatoes, egg- 

FIG. 80. The peculiar cell divisions by which eggs and sperms are formed. 
(Redrawn from Robbins and Pearson in Sex in the Plant World.) 

plants, English forcing cucumbers, oranges, grapes, and bananas 
are quite common examples. 

Problem 2. How is pollen dispersed? 

We have learned that the pollen grain, when shed by the 
anther, usually consists of a two-celled male plant, enclosed by 
a thick, protective wall. The pollen grains of flowering plants 
differ greatly in size, shape, and surface markings. Inasmuch 
as plant pollens are responsible for much of the hay fever, there has 
been much interest in them, and some investigators have become 
proficient in identifying them under the microscope. 

Exercise 89. Different kinds of pollen grains. Examine, under the 
compound microscope, pollen from a variety of plants, including such common 
hay-fever plants as ragweeds, Russian thistle, pigweeds, grasses, oak, black 
walnut, poplars, and elms. Also observe the winged pollen grains of pine. For 



examination of pollen grains, mount them dry on a slide, or in a mineral oil and 
cover with a glass slip. When mounted in a watery and most other liquid 
media, the grains either shrink or swell, or become distorted. 

Quantity of pollen. The amount of pollen given off by plants 
is enormous. One worker counted 243,000 pollen grains, the out- 
put of a single dandelion blossom. This same worker estimated 
that an entire rhododendron plant produced approximately 

FIG. 80. The pollen-bearing catkins of walnut. 
The catkins are easily swayed by the wind, and 
the pollen is light in weight and produced in 
abundance. These characters make the plant 
well adapted to wind pollination. (Photograph 
furnished by Division of Pomology, California 
College of Agriculture.) 

FIG. 82. The cleis- 
togamous flowers of 
closed gentian. The 
flowers never open; 
hence only self polli- 
nation can occur. 

72,620,000 pollen grains. It is said that a medium-sized Indian 
corn plant will produce as many as 50,000,000 pollen grains. 
Gager says: " It was calculated that between 8 A.M. and 1 P.M. 
on a certain day there were given off from a single plant of Am- 
brosia trifida (ragweed) the amazing number of eight thousand 
million (8,000,000,000) pollen grains." 

Agents which disperse pollen. Pollen is carried chiefly by 
wind and insects. Even when male and female organs are borne 



in the same flower, as they usually are, outside agencies are most 
always depended upon for pollen transportation. In fact, there 
are only about 150 species of flowering plants which do not need 
pollinating agents. These are the cleistogamous flowers (from 
cleisto-closed + gamos-marriage). Their flowers never open, and 
their pollen tubes grow directly from the stamens into the pistils. 
Certain violets are an example of such flowers. 

Such inconspicuous flowers as those of grasses, cottonwoods, 
alders, birches, oaks, hickories, and pines are notable among those 

which have their pollen 
dispersed by the wind. 
With the exception of 
the nut fruits, the com- 
mon tree fruits are largely 
dependent upon insects 
for the dispersal of their 

In grasses the flowers 
are inconspicuous, they 
lack odor and nectar, 
and hence are unat- 
tractive to insects; 
furthermore, the pollen 
is light and dry, and 
easily blown; the stigmas 
are feathery and expose 

FIG. 83. Staminate flowers of cottonwood. 

Wind carries the pollen from the cottonwood 

tree bearing staminate flowers to the pistils 

of the tree bearing pistillate flowers. 

a large surface to fly- 
ing pollen; and pollen 
is often produced in 

great quantities. For example, in corn, it is estimated that 
each staminate flower group (tassel) produces 20,000,000 to 
50,000,000 grains of pollen. There are in the neighborhood of 
45,000 pollen grains produced for each ovule. The styles, the corn 
" silks," are long and plumose, and are receptive throughout their 
entire length. Pollen grains of wind-pollinated flowers are often 
much roughened. What is the advantage of this to the plant? 

In cottonwoods, alders, birches, oaks, and hickories, the flowers 
are in catkins. The staminate catkins are pendulous and move 



easily in the wind, and the light pollen is shaken from the anthers 
and readily carried away by the breezes. In many catkin-bearing 
trees the flowers open before the leaves unfold so that pollen move- 
ment is unhampered. 

In pines, the flowers are also borne in short catkins; and, in 
addition to this feature which favors wind dispersal of pollen, the 
pollen grains themselves are provided with two wings which 
assist in their distribution by the 
wind. In pines, pollen is produced 
in tremendous quantities. At the 
proper season, showers of pollen 
may be witnessed in the pine 
forest; one's clothing may become 
yellow with the pollen grains. 

The principal pollinating in- 
sects are bees, the most efficient 
of which are the honeybee and the 
bumblebee. It is known that 
French and sugar prunes in Cal- 
ifornia and Napoleon and black 
Tartarian cherries set a very light 
crop unless a large number of bees 
are present in the orchards at 
the time of blooming. In fact, 
insects are necessary for the pol- 
lination of most deciduous fruit 
trees except certain nuts. 

The flowers of red clover must 
be cross-pollinated in order to set 
seed on a commercial basis, and 
the bumblebee is chiefly respon- 
sible for carrying the pollen. This insect is capable of pollinating 
30 to 35 clover flowers a minute. Have you noticed that bees in 
their work confine themselves, for the most part, to visitation of 
the flowers of one species? What is the advantage of this to the 

In many types of figs, including Smyrnas, but excepting the 
common black fig, all or at least one of the crops require the visi- 

FIQ. 84. Timothy in bloom. The 
grasses have flowers that are not 
showy or fragrant. They are fitted 
by structure and position to wind- 


tation of the fig wasp, bringing with it pollen, for the fruit to form 

The moths and butterflies are also important pollinating agents. 
They are particularly adapted with their long mouth-parts to secur- 
ing nectar from flowers with long tube-shaped corollas, such as 
larkspurs, columbine, and nasturtium. 

Insects are attracted to flowers chiefly by their odor and color. 
Odor appears to be the more important influence. Many flowers 

FIG. 85. Hives of honey bees in an orchard. The insects carry pollen from 

flower to flower, thus bringing about a better setting of fruit. (From Division 

of Pomology, College of Agriculture, University of California.) 

have special nectar-secreting structures known as nectaries or 
nectar glands. Sugar is the main secretion of these glands. In- 
sects also visit flowers in search of pollen, which is used as a food 
mainly for the larvae. 

In general, insect-pollinated flowers have both stamens and 
pistils in the same flower; the stamens usually have short fila- 
ments, the flower groups are quite inflexible, the pollen is often 


sticky and produced in relatively small quantities, and the flowers 
are attractive because of their showiness or odor. 

Enumerate the features which are favorable to insect pollina- 
tion. To wind pollination. 

Compare insect- and wind-pollinated plants as to waste of 

How do you account for the fact that house plants often set 
less seed than plants growing out-of-doors? 

Longevity and viability of pollen. Pollen varies considerably 
in the length of time it will remain viable (capable of germination), 
depending upon the moisture and temperature conditions sur- 
rounding the grains, and upon the kind of pollen. 

Corn pollen does not remain viable much longer than 24 hours 
after shedding. That of Hibiscus trionum lives no longer than 
3 days. Pollen of the date palm will retain its viability for several 
months, if kept dry. The longevity of apple pollen has been 
variously reported by different investigators. One worker 
records germinations of 12, 10, 5, and 8 per cents for different lots 
after 7 months of storage in the laboratory, with a temperature 
ranging from 50 to 65 F. The pollen of apple and plum remains 
alive much longer if stored in closed vessels which prevent drying 
out than when stored in the open. The pollen of some plants, 
such as sugar beet, alfalfa, and red clover, absorb water rapidly 
and burst in water or in a saturated atmosphere. Such pollen 
loses its viability rapidly in an atmosphere of high relative 

Dry pollen will withstand greater temperature extremes than 
moist pollen. However, resistance to low temperature is also a 
specific character. For example, pollen of apple, pear, and plum 
will withstand temperatures ranging from 33 to 34 F., whereas 
about 50 per cent of peach and apricot pollen grains are killed by 
this temperature. 

Immediate effect of pollen. It is noticed that shortly after 
pollination the stigma withers. This is the immediate effect of 
pollination. After a time the petals also wither and drop off. If 
flowers are bagged and pollination prevented, the petals remain 
fresh for a much longer time than they do in pollinated flowers. 



Problem 3. What are the important different types of flowers? 

In Exercise 85 we learned the principal parts of a typical 
flower. These are as follows: the sepals, the petals, the stamens, 
the pistil. Such a flower is said to be a complete flower. But 
not all flowers have these four sets of floral organs; one or more of 


FIG. 86. Garden asparagus (Asparagus officinalis). A, pistillate flower; 

B, staminate flower; C, mature fruit; D, section of fruit; E and F, portions 

of the plant showing method of branching, position of flowers and leaves. 

(From Robbins, in Botany of Crop Plants.) 

these sets may be lacking, in which case the flower is said to be 

Incomplete flowers. In the buckwheat flower, for example, 
the petals are absent. In the flowers of willows and cottonwoods, 
both sepals and petals are lacking. In both of the foregoing cases 


the essential organs (stamens and pistil) are present. However, 
some flowers have but one set of essential organs, either stamens 
or a pistil. A flower with stamens only, and no pistil, is said to 
be staminate (male). On the other hand, a flower with a pistil 
but no stamens is said to be pistillate (female). Staminate plants 
do not bear fruit and seed; only pistillate plants perform this 
function. Staminate and pistillate flowers may be on the same 
individual plant; this is true of corn, in which the " tassel " is a 
group of staminate flowers and the " ear " a group of pistillate 
flowers. The squashes, pumpkins, and melons are other examples 
of plants which bear staminate and pistillate flowers on the same 
plant. Or staminate and pistillate flowers may be on different 
individual plants; examples of such plants are asparagus, spinach, 
hops, willows, and date palm. In these plants the flowers on any 
one plant are either all staminate or all pistillate. Thus we may 
speak of staminate (male) plants and pistillate (female) plants. 

In certain cultivated species which have male and female indi- 
viduals, one of the two kinds of plants may be more desirable 
from the grower's standpoint than the other. For example, in the 
date palm it is desirable that most of the individuals be pistillate 
since these alone can bear the edible fruit. In the hop plant, it is 
only from the pistillate plants that the " hops " are obtained. In 
asparagus, it has been found that the yield of edible spears from 
staminate plants exceeds that from pistillate. Staminate cotton- 
woods are preferred to pistillate ones because of the " litter " 
caused by the cotton-covered seeds. 

Exercise 90. Incomplete flowers. Study the incomplete flowers of such 
plants as pumpkin, spinach, asparagus, willow, cottonwood, and begonia. 
What is a staminate flower? A pistillate? Why are staminate cottonwood 
trees better as street trees than pistillate ones? How can you propagate 
staminate individuals? Will a solitary asparagus plant produce fruit? A 
solitary cottonwood? A solitary hop-vine? Explain why. 

Lily type of flower. The lily family includes such well-known 
plants as the lily, yucca, hyacinth, tulip, onion, and asparagus. 
In this family, the parts of the flowers are in threes. The non- 
essential organs consist of six separate parts, in two circles of three 
each, which are generally very similar in size, shape, and color. 
The anthers are usually large and conspicuous. The ovary is 



FIG. 87. Lilium graridiflorum, a 

FIG. 88. The inflorescence 
(umbel) of leek, a plant closely 
related to onion. At the top 
and right is a flower-group still 

2(J glume 

/if glume 

Fia. 89. Spikelet of common panicle oats, 

of Crop Plants.) 

(From Robbins, in Botany 



divided into three chambers, each of which commonly has several 
seeds. Flowers of the lily type are chiefly insect-pollinated 
Name ten plants of economic importance belonging to the lily 

Grass type of flower. The flower of the grass family 
(Gramineae) is peculiar. It may be studied to advantage in such 
common grasses as wheat, oats, barley, and rye. In all grasses, 
the flowers are in groups, each group being called a spikelet. A 
typical spikelet, such as 
that of oats or wheat, con- 
sists of a short axis, bear- 
ing a number of chaff-like 
bracts. The two lowermost 
bracts, called glumes, are 
empty, that is, do not bear 
flowers in their axils. 
Above the two glumes are 
one or more bracts called 
lemmas, and usually there is 
a flower in the axil of each. 
Each flower consists of three 
stamens and a single pistil. 
The ovary contains a single 
ovule and has two feathery 
stigmas. The awns or 
beards of a grass are brittle 
structures usually attached 
to the lemmas. Do you 
think that inconspicuous 
flowers of the grass type, 
with their lack of showy parts and nectar glands, are wind- 
pollinated or insect-pollinated? Explain. 



Fia. 90. Wheat flower with lemma re- 
moved; considerably magnified. (From 
Robbins, in Botany of Crop Plants.) 

Exercise 91. Dissect the spikelets of oats, wheat, or barley, and find the 
parts described in the previous paragraph. Write a short paper on the topic 
" Grasses and Man." 

Mustard type of flower. The mustard family (Cruciferae) 
includes a number of familiar plants such as cabbage, turnip, 



rutabaga, rape, mustard, radish, watercress, and horseradish, and 
a number of pernicious weeds such as pennycress, wild mustard or 
charlock, shepherd's purse, false flax, and tansy mustard. The 
mustard flower is characteristic. It has four sepals, four petals, 

six stamens (two short and 
four long), and a two- 
celled ovary. The four 
petals are so arranged that 
when one looks at the face 
of the flower it has the ap- 
pearance of a Greek cross, 
hence the name Cruciferae 
(Latin, crux, cross, +fero 
bear). The pistil has a 
single style with a more or 
less two-lobed stigma. 
Insects are the principal 
agents in the pollination 
of mustard flowers. Why? 
Name ten plants of eco- 
nomic importance belong- 
ing to this family. 

Rose type of flower. 
The family Rosaceae in- 
cludes such plants as the 
raspberry, blackberry, 
dewberry, strawberry, 
spiraea, and rose. The 
flowers are generally com- 
plete, except in some culti- 
vated varieties of straw- 
berries. There 'are usually 
five sepals and five petals. 
In most cultivated roses, 
which have double flowers, 

there are numerous petals which have developed from young 
tissue that normally becomes stamens. Except in these double 
sorts there are numerous stamens, and as a rule, a number 

FIG. 91. Corn (Zea mays). Young pistil- 
late inflorescence ("ear"), showing the long 
styles ("silks"). (From Robbins, in Botany 
of Crop Plants.) 



of separate pistils. The rose type of flower is chiefly insect- 

Apple type of flower. The apple, pear, quince, loquat, and 
service berry are members of the apple family (Pomaceae). This 
family bears flowers which are complete and usually have a concave 
or cup-shaped receptacle, to which are attached a five-lobed or 
five-toothed calyx, five separate petals, numerous distinct stamens, 
and a one- to five-celled ovary. 
Pollination of the apple type of 
flower is brought about by insects. 

Plum type of flower. The 
plum family (Drupaceae) includes 
the plum, cherry, almond, peach, 
and apricot. This is a group com- 
monly known as the stone fruits. 
The flowers are complete. The 
corolla and calyx each have five 
distinct parts. There are nume- 
rous stamens. In a longitudinal 
section of the drupaceous flower 
it is seen that the ovary is placed 
down within a cup commonly 
called the " calyx tube." There 
is one pistil situated at bottom of 
the hollow receptacle, and a one- 
celled ovary, usually maturing 
one seed. Pollination of the plum 
type of flower is chiefly by in- 

Legume type of flower. The 
pea family (Leguminosae) is one 

of wide geographical distribution and possesses a great many 
species. Well-known representatives are common garden pea, 
vetch, sweet pea, clovers, sweet clovers, alfalfa, bean, cow-pea, soy 
bean, and peanut. The flowers are irregular in form; they have a 
butterfly-like shape. The calyx is usually four- or five-toothed. 
The petals are normally five in number, a broad upper one 
(standard), two side ones (wings), and two lower ones more or less 

FIG. 92. Flower of mustard. 
Diagram of flower above, and 
flower in median lengthwise sec- 
tion below. (From Robbins, in 
Botany of Crop Plants.) 

nm of receptacle/ 

FIG. 93. Median longitudinal section of apple flower. (From Robbing, in 
Botany of Crop Plants.) 

FIG. 94. Median lengthwise sec- 
tion of the flower of sour cherry. 
(From Robbins, in Botany of Crop 



united along one edge, forming the keel; this keel incloses the 
stamens and pistil. Stamens are usually ten in number; com- 
monly nine are united and one is free. There is a single pistil, 
with one cell. Some of the legumes, such as the garden pea, are 
self-pollinated; many others are pollinated by insects. 

Composite type of flower. The thistle or composite family 
(Compositae) possesses a number of well-known plants, among 
which are common lettuce, Jerusalem artichoke, endive, salsify, 
dandelion, yarrow, sage, chrysanthemum, sunflower, golden rod, 

FIG. 95. The choke cherry 

has its flowers in long 


FIG. 96. Flower structures of the 
pea or legume family. Left, the 
calyx and corolla have been re- 
moved, exposing the stamens (10) 
and the style. Nine filaments are 
united at base to form a tube 
which surrounds the ovary; 1 
stamen is free. 

sow thistle, dahlia, aster, marigold, fleabane, everlasting, Spanish 
needles, and thistle. In this family the individual flowers are 
grouped to form a flowerhead. A " sunflower " is not a single 
flower, but a group of individual flowers, mounted on a common 
receptacle. As a rule, in the flower head, there are two kinds of 
flowers: (1) those about the margin, called ray flowers; and (2) 
those in the center, known as disk flowers. In such composites 
as lettuce, however, all the flowers of a head are alike. The disk 



flowers have a calyx made up of bristles or scales. These are 
attached at the top of the ovary. The corolla is tube-like, and on 
its side are attached the five stamens. There is a single pistil, 
which has a one-seeded ovary, and a single style. The ray 



FIG. 97. Pollination of alfalfa. A, flower untripped with calyx and stan- 
dard removed; B, same tripped; C, position of staminal tube untripped 
and tripped. (After U. S. Dept. Agr. from Robbins, in Botany of Crop 


flowers are usually imperfect. Insects are the principal agents in 
the pollination of composite flowers. 

Double flowers. Many cultivated plants tend to develop 
double flowers. Well-known examples are forms of dahlias, chrys- 
anthemums, pinks, roses, and hollyhocks. Doubling may arise 



through the change of stamens or pistils to petals, or through the 
origin of extra petals in the circle of petals. 


^ fkWrj 


ovary wall 

FIG. 98. Jerusalem artichoke, a member of the composite family. A, 
lengthwise section of the flowering head, Xl; B, ray flower, X6; C, disk flower, 
cut lengthwise, X6. (From Robbins, in Botany of Crop Plants. A after 


Exercise 92. A study of flower types. Study the following types of 
flowers: mustard, rose, apple, plum, legume, composite, and double. These 
should be dissected. Prove to your own satisfaction that each of the flowers 
studied illustrates the features of the type to which it belongs. 

Problem 4. What are the principal causes of the failure of 
blossoms to set fruit? 

There are often reproductive failures in plants. They may 
bear an abundance of blossoms, but owing to one or more causes, 


fail to set fruit. Among these causes may be mentioned the fol- 

1. Pollen is not shed at the time when the stigmas are recep- 
tive. The pollen may be shed before, or after, the stigmas are 
receptive. In some American plums, particularly during periods 
of cold weather, the stigma may pass the receptive condition before 
the pollen is mature. What visible evidence is there that a stigma 
is ready to recieve pollen? 

2. Pollen is not viable. Some cultivated varieties of grapes 
bear impotent pollen. Certain varieties of strawberries (Glen 
Mary and Crescent) produce impotent pollen and hence are self- 
sterile. Several commercial varieties of peaches, notably Chinese 
Cling and J. H. Hale, produce no good pollen. They are male 
sterile. Of course, these varieties will not set fruit unless inter- 
planted with varieties which will furnish pollen. 

3. Imperfect flowers. There is a commercial sterility problem 
with strawberries involving chiefly the impotence of the pistils of 
the perfect flowers. Certain varieties of strawberries develop only 
perfect flowers, and all flowers are fertile. Other varieties have 
more or less female sterile perfect flowers, and still others bear 
only pistillate flowers. If such a variety as the last mentioned 
is planted by itself, there will be no pollen. In planting varieties 
with pistillate flowers only, it is necessary to have rows nearby 
planted to pollen-bearing individuals. 

4. Self-sterility. Many varieties of orchard fruits are not 
capable of setting fruit unless pollen from another variety is used. 
That is, they are self-sterile. For example, the Montmorency 
cherry is self-sterile but may be cross-pollinated by Early Rich- 
mond or English Morello. In some localities the Spitzenburg 
apple is self-sterile, but can be fertilized with pollen from a number 
of other varieties, such as Yellow Newton, Arkansas Black, Jona- 
than, and Baldwin. Evidently, the mutual affinities of varieties 
must be considered in setting out an orchard. It would not be 
well to plant solid blocks of Spitzenburg apple, for example. There 
should be, here and there in the orchard, trees of some one of the 
other varieties, the pollen of which is capable of fertilizing it. 

Self -sterility in pears is the reason for the barrenness of many 
pear orchards. It has been frequently observed in many parts of 



Copyright of Journal of Heredity 

FIG. 99. Certain varieties of strawberry bear only hermaphroditic flowers 

(above). Other varieties of strawberry bear only pistillate flowers (below). 

(After Darrow, from Robbins and Pearson, in Sex in the Plant Woild.) 


the country that when a certain variety of pear is planted in a 
solid block, a pronounced failure to set fruit often results. This is 
particularly true, it seems, of Bartlett and Kieffer pears. These 
varieties give much better results when they are planted with such 
varieties as Lawrence, Duchess, and Anjou. 

5. Excessive production of flowers. Many plants initiate 
development of far more flowers than they can perfect; and often 
plants cannot mature all the fruits that set from those flowers 
which are perfected. The " June drop " of the immature fruits 
of certain fruit trees is due to the abortion of embryos which the 
trees have not reserve food enough to mature. 

6. Unfavorable weather conditions. Fruit-setting may some- 
times fail because of frost or because of cold, rainy weather which 
interferes with the movement of insects or delays the growth of 
the pollen tube; or hard rains which come immediately after the 
pollen is brought to the stigma may wash the grains from the 
stigmas. In the case of corn, hot dry winds may wither the 
" silks," making it impossible for the pollen to stick to them and 
germinate; as a result there is an incomplete " filling " of ears. 
What are the " silks " of corn? The tassels? Is corn wind- or 
insect-pollinated ? 

7. Lack of pollinating insects. In most of our orchard trees 
the pollen is carried by insects, chiefly bees. It has been shown 
that under certain conditions the percentage of flowers setting fruit 
can be increased by placing beehives in the orchard. It is usually 
considered that one hive of bees to one or two acres of orchard is 

There are other types of sterility among flowering plants. 
Abortion is a rather common occurrence in the plant kingdom. 
For example, in the coconut normally there are three sections in the 
ovary, but, while it is maturing, two of the sections abort ; likewise 
in the date palm, which also has three sections in the ovary, only 
one section becomes a fruit; in oaks there are three sections in the 
ovary, each with two ovules, but the mature acorn has but one 
seed; in plums, cherries, almonds, and other stone fruits, the 
ovary normally has two ovules, but only rarely do both oi these 
develop into seeds. In fact, when two seeds are found in a cherry 
it is occasion for special remark. 


Several cultivated plants develop but a small number of flowers 
or only poorly developed ones. For example, some Irish potato 
varieties produce very few flowers; the same is true of the sweet 
potato, in which seeds may be almost lacking. In France, garlic 
seldom flowers. Sugar cane is a notable example of a plant which 
seldom produces seeds. How is a plant like sugar cane propa- 
gated? What is the advantage of reproducing potatoes by means 
of tubers rather than seeds? 

Problem 5. How do ferns and mosses reproduce? 

Among ferns, as among seed plants, the sexual function is 
performed by minute, very peculiar sexual plants. But in ferns 
there is no embryo sac in an ovule of the ovary, no masses of yellow 
pollen to be carried about by bees or blown in clouds by the winds. 
There are, instead, flat green growths hidden under the forest moss 
and mold. The largest kinds of sexual plants in ferns are seldom 
half an inch across; the smallest of them grow to maturity and 
produce their eggs and sperms all within the protective coat of the 
single spore from which they grew. 

Exercise 93. How do ferns produce spores? Ferns have no flowers or 
stamens or pistils. Look on the under side of almost any fern frond in late 
summer or autumn and notice there the small, regularly arranged brown 
warts. Each of these consists of a group of spore cases (sporangia), in many 
instances protected by a covering of thin tissue (the indusium) . Each group of 
spore cases is called a sorus (plural, sori). Mount some of the spore cases in 
water, and examine with the compound microscope. The spore case or sporan- 
gium is watch-shaped, with a thick wall about one edge, thin side walls, and a 
stalk. Inside are the spores. Give important differences between a spore 
and a seed. 

In moist, protected soil, the spores germinate, each growing into 
a miniature green plant of one sex or the other, or even into plants 
bearing the organs of both sexes. Most fern sexual plants are 
peculiar flat growth like bits of leaves, heart-shaped and about 
J- to -- inch across. The sexual plants of ferns are called 'prothallia 
(singular, prothallium). 

Exercise 94. How do ferns produce sex cells or gametes? On the moist 
soil beneath ferns, look for the prothallia of ferns. An experienced greenhouse- 
man will always be able to find them for you. Study them with the dissecting 



microscope. Observe that they have root-like threads (rhizoids) fastening 
them to the soil. Examine prepared slides showing the sexual organs. 

The male organs the structures producing sperms are among the 
rhizoids; the female organs the structures producing eggs are clustered near 
the notch of the heart-shaped plant. A male organ is a minute spherical mass 
of cells protruding below the under surface of the prothallium. The inside 


of egg in 

by sperm 



(Protha Ilium) 

Spores Develop 


Breaks open 
-Spores are 
after Tetrads 
Break up 


Tetrads of 5pores 
are Formed unthin 


and Dies 
as Young 


Developed Sporophyte 
or Pern Plant 

Gametophyte Generation * 5porophyte Generation 

FIG. 100. Life history of a fern. Note that the cycle is made up of a sporo- 
phyte generation which includes the showy fern plant, and a gametophyte 
generation which includes structures that are small and not frequently seen 

in nature. 



cells of the male organ grow into long, coiled sperms, out of which curious long 
arms of protoplasm grow. With these arms, the sperms swim agitatedly 
about in the moisture under the prothallium until they reach the chimney-like 
structure of the female organs. A female organ is a flask-shaped structure, 

FIG. 101. Fern fronds showing peculiar " warts " on the under sides. These 
are spore clusters. (From Robbins and Pearson, in Sex in the Plant World.) 



the base of which is a chamber in which there is a single egg. The " neck " 
of the flask-shaped female organ is a chimney-like structure. The sperms swim 

FIG. 102. Two of the " warts " of Figure 101 highly magnified. These 
" warts" are essentially clusters of minute balls on stems. The balls are full 
of spores. (From Robbins and Pearson, in Sex in the Plant World.) 

through these chimneys and fertilize the egg. Fertilized eggs grow out of the 
female plant, developing directly into the beautiful fern fronds we know. 

Fia. 103. Fern prothallia in the laboratory. Fern spores may be germinated 

in the laboratory if sown on damp inverted flower pots standing in a solution 

of small quantities of the salts required for their germination and growth. 

Exercise 95. What is the nature of the leafy moss plant? Examine indi- 
vidual moss plants taken from a mossy turf. Each plant consists of a slender 



stem or axis, bearing many 
tiny overlapping leaves. 
The plant is anchored to 
the soil by rhizoids which 
not only serve as hold- 
fasts, but also absorb mois- 
ture. If moss leaves are 
mounted flat in water and 
examined with the com- 
pound microscope, it will 
be seen that they are very 
thin, often no more than 
one cell thick. 

The asexual moss 
plant. The familiar 
mossy turf is the sex- 
ual generation of the 
moss plant; and in- 
stead of plants with 
leaves and stems and 
roots producing spores, 
like seed plants, the 
spore-producing moss 
plant has only a small 
brown capsule on a 
stalk growing directly 
out of the female sex 
organs and feeds as a 
parasite on the sexual 
plant. In the green- 
house or woods one can 
find these small cap- 
sule plants growing out 
of the mats of moss. 

Exercise 96. How do 
mosses produce spores? 

Study moss plants bearing 
the capsules (sporangia) 
each on a long stalk. The 
capsules or sporangia are 

FIG. 104. Moss. A, portion of leafy moss plant 
and spore-bearing structure; B, single spore 
showing germ tube; C, algal growth which de- 
velops from germinating spore; D, female sex 
organ; E, male sex organ. (From Holman and 
Robbins, in A Textbook of General Botany.) 



bears the eqq 

produces sperms 

Sporophi^fe VH 

Spore plant develops 
from the fertilized eqq, 
qrows from the leaf q 
plant and produces 
spore "tissue 

Leafif plants develop 
from buds and develop 
sex orqans 


Spore qermi nates and 
produces a protonema 
with buds 

Spores escape 
from capsule of 
the spore plan* 

FIG. 105. Life cycle of a moss plant, showing alternation of generations. 
Note that the spore plant results from the union of gametes and that the 
gamete plant results from the germination of a spore. How does the gameto- 
phyte of the moss get its food? What is the source of food of the sporophyte? 
Of what importance is chlorophyll in the life cycle of the moss plant? In what 
way is the gametic reproduction of moss different from the gametic reproduc- 
tion of bread mold. What is the name given to the union of two unlike 
gametes resulting in the formation of a zygote? 


nothing more than powder boxes of spores. Break open a mature capsule in a 
drop of water and examine the spores. When these spores germinate they 
develop into green moss plants. The early stages in the growth of moss plants 
from spores are green branching threads, growing close to the moist soil, and 
often giving to it a greenish color. 

The sex organs of mosses look much like the sex organs of 
ferns. However, they are not embedded in the leafy sexual plant, 
and the male organs are larger and stalked and the female organs 
have longer necks. They are borne in clusters in the hearts of the 
buds at the tips of the moss branches. Their sperms can swim to 
the eggs in the slightest bit of moisture perhaps a drop of dew. 

Suggested activities. What is the importance of mosses in nature? 
Write a report on the importance of sphagnum and other mosses to man. 

Problem 6. How do plants reproduce asexually? 

The starfish, which is such a pest because it feeds on oysters, 
and the quackgrass, which chokes out crop plants and which the 
farmers of the country annually spend thousands of dollars to 
control, can not be killed by chopping up. Each dissevered arm 
of the starfish grows a whole new starfish; each stalk of quack- 
grass grows a whole new plant. Only a few simple animals can be 
multiplied by breaking them into pieces, but think of the many 
complex plants which can be reproduced in this asexual " vegeta- 
tive " manner. Cuttings of stems of many roses, willows, apples, 
and a host of other plants will develop roots and grow if kept in 
moist soil. The strawberry sends out runners stems with buds 
at the end which root and form new strawberry plants in much 
the same fashion as the interesting " walking fern," except that 
in the fern it is the tip of a leaf from which the new plant grows. 
Begonia leaves will send out roots and stems and leaves if pinned 
closely to moist soil. Potato tubers and lily bulbs are also special 
asexual reproductive structures. 

Exercise 97. The gemmae of Uvei worts. The liverworts, common in 
greenhouses and moist woods, are simple flat green growths, with distinct 
under and upper surfaces. Some kinds of them are much larger than most 
mosses and look like green snakeskin. They reproduce both sexually and 
asexually. Observe on the upper surface of certain plants minute cups full of 
very small green pieces of liverwort tissue no larger than the head of a pin; 



these are called " gemmae." Currents of air scatter these on the moist soil 
and they grow directly into new liverwort plants. The plant has reproduced 

Reproduction by fission. When a bacterium reproduces it 
splits across the middle into two bacteria. Reproduction here is 
nothing more than cell division. The cell in this case is a minute 
system which can grow and split into two systems, each capable 

of growing and splitting across 
the center into two more systems, 
and so on ad infinitum. This 
simple type of reproduction is 
called fission. It occurs among 
more complex one-celled organ- 
isms than bacteria. 

The blue-green algae also re- 
produce by fission. Many kinds 
grow in hot springs, lining them 
with bright-colored crusts. Others 
grow in soil. Some are very 
troublesome in reservoirs, impart- 
ing a disagreeable flavor to the 
water. Also certain green algae 
reproduce by fission. 

Exercise 98. Asexual reproduction 
in Protococcus. This is a one-celled 
plant which may be found growing on 
the north side of trees, on old damp 
boards, and on foundations of buildings. 
It forms a growth which resembles 
green paint. Scrape off some of the 
material with a knife and mount in 
water on a slide. Each cell is a sepa- 

FIG. 106. Walking fern showing 
vegetative reproduction, and migra- 
tion of the plant. 

rate plant. It will be observed that reproduction is accomplished by the 
division of the whole body of the parent. 

Reproduction by spores. Everyone is familiar with the white 
threads of the " bread mold/ 7 which grows, as the name implies, 
on stale bread. The small black spherical bodies on stalks which it 
sometimes produces contain the asexual spores. These are in the 



air, even in the best-regulated kitchen, and when in abundance 
produce the well-known musty odor. 

Exercise 99. Asexual reproduction in bread mold. Moisten stale bread 
and place under a bell jar or other cover. Within a few days the surface of the 
bread will be covered with a cottony growth, and after a time there will appear 
numerous black bodies. These are spore cases or sporangia. Each sporangium 
contains thousands of spores. Examine with high-power dissecting micro- 
scope. Also mount spore cases in water, and study with the compound 
microscope. The significant feature of the asexual method of reproduction in 
bread mold is that the spores are capable of growing directly into new bread- 

Fia. 107. Cupfuls of tiny liverworts grow on the upper sides of old liverworts, 
odd relatives of the mosses. (From Robbins and Pearson, in Sex in the 

Plant World.) 

mold plants. There is, however, sexual reproduction in bread mold, but the 
asexual method is by far the more important. (See pp. 101, 103.) 

Slime molds are simple plants which creep about, streaming 
masses of slime, in rotting logs and stumps. Single slime-mold 
cells creep about and reproduce freely by fission. But besides 
fission these slimy masses have another method of asexual repro- 
duction; this method is spore formation. 

A spore is merely a living cell which seems to find it necessary 


to escape from its mother that it may grow into a new plant. 
When slime molds initiate reproduction by spores, erect growths 
with solid walls appear on the slimy mass, and grow into small odd- 
shaped powder puffs of threads and heavy-walled spores. If 
these spores eventually settle in a sufficiently moist place, they 
open and release more spores. This time the spores can swim. 
They swim about for a time in the wet leaf mold or soggy wood, 
then again flow about. 

In the seaweeds, in mosses, ferns, and seed plants, there are 
asexual spores, that is, spores which are capable of growing directly 
into a new plant without union with any other mass of living 
material. For example, in mosses asexual spores are borne in a 
capsule (sporangium), and these spores grow into the moss plant, 
which in turn develops eggs and sperms. In ferns, asexual spores 
are borne in cases which, grouped together, form the brown 
" warts " on the back of fern fronds. These spores germinate, 
developing a small heart-shaped structure, which in turn is the 
bearer of male and female organs. In seed plants there are also 
asexual spores, as well as sexual bodies. There are small asexual 
spores in the anthers which develop into pollen grains (male 
plants) ; and large asexual spores in the ovules which develop into 
embryo sacs (female plants). 

Problem 7. How are plants propagated artificially? 

The multiplication of plants artificially is commonly known as 
propagation. Among our common seed plants there are two dis- 
tinct methods of propagation: (1) by the use of seeds, and (2) 
by the use of some vegetative organ, such as stem, or root, or leaf. 

We have learned that in the seed there is an embryo, or young 
plant, and that it is formed as a result of the process of fertilization. 
The embryo of the seed is a result of the union of two sex elements. 
One of these elements, the pollen grain, may or may not have come 
from the plant which bears the seed; if it did not, then the embryo 
in the seed has characters of two parents. This means that, when 
a plant is cross-fertilized, the resulting seeds will produce plants 
in many particulars unlike the parent which bears them. Propaga- 
tion by seeds is a sexual method. 



On the other hand, when a plant is propagated by means of 
stems or roots or leaves, we may be certain that the parts used 
will produce plants closely resembling those from which they were 
taken. This method of propagation is vegetative. 

Let us cite an example which further distinguishes between the 
methods of propagation by seeds and by vegetative organs. The 
pollen of strawberries is carried chiefly by bees; hence there are 
great chances that the ovules of any particular flower will be fer- 

FIG. 108. Modern method of propagating the tropical orchid. Formerly 

propagated in cultivation only by vegetative means, orchids are now grown 

from seed germinated on specially prepared media. 

tilized by pollen from another plant. If they are so fertilized, the 
embryo which results will possess characters of two parents. 
This accounts for the fact that strawberries when grown from seed 
seldom come true to type ; that is, they seldom are like the parent 
plant from which the seed was taken. If we wish to propagate a 
desirable variety of strawberry and keep it " true," we use the 
" runners " or vegetative parts. Runners are merely " chips off 
the old block." 

Propagation by the use of vegetative organs is practiced if the 



plants do not come true from seed. This is the case with hybrids 
and many horticultural varieties. Also a number of plants such as 
sugar cane, banana, sweet potatoes, and Irish potatoes seldom 
produce seed and, of course, must be propagated vegetatively. 

In propagating plants by vegetative means, use is made of 
steins, of roots, and of leaves. Many more plants can be vegeta- 
tively propagated by stems than by roots or leaves. (A. stem struc- 
ture usually may be readily distinguished from a root as follows: 
The stem is divided into definite joints and the buds, hence, the 
branches arise in regular order; the buds occur in the axils of the 
leaves. Roots, on the other hand, do not have definite joints, and 

usually bear no buds or leaves. 
Relatively few plants can be 
propagated by means of their 
roots. Among them may be 
mentioned the sweet potato, 
dahlia, raspberry, and certain 
blackberries. Rarely, the leaf 
may be employed as a prop- 
agating organ. A striking 
example is seen in begonia 
and Bryophyllum. 

Propagation by separation. 
In this process of propagation 
use is made of such vegetative 
parts of the plants as become 
detached naturally. These in- 
clude such structures as bulbs, bulblets, bulbels, corms, and tubers. 
Each of these possesses one or more buds which, under proper 
conditions, are capable of growth. 

Exercise 100. Propagation by separation. Study bulbs, bulbels, bulblets, 
corms, and tubers. These should be brought to the laboratory, their structure 
studied, and some set out under conditions favorable for growth. Talk with a 
plant propagator in a greenhouse concerning methods of growing them and 
their use in multiplying plants commercially by separation. 

Bulb. A splendid example of the bulb is the onion. Examina- 
tion of the mature bulb of the common onion shows it to be made 

FIG. 109. Vegetative reproduction of 
Tolmiea menziesii from a leaf. 


up of the much-thickened bases of leaves, attached to a small cone- 
shaped stem. These scale leaves are quite rich in food material. 
The bulb possesses a terminal bud and occasionally lateral buds in 
the axils of the leaves. 

Bulbel. Often a number of small bulbs will develop about a 
large mother bulb. These are called bulbels. For example, in 
the white lily (Lilium candidum) a group of bulbels is formed at the 
top of the mother bulb, and each produces a number of roots. 
These bulbels may be separated from the mother bulb, and each 
used to propagate a new plant. 

Bulblet. These are small bulbs which are developed above 
ground, usually in the axils of leaves or in the flower group. They 
do not differ essentially from bulbels except in their position on the 
plant. In the tiger lily (Lilium tigrinum) small bulbs occur in 
the axils of the leaves; they may be removed from the parent plant 
and used for the purpose of propagation. In " top/' " tree/' or 
Egyptian onions, clusters of bulblets are developed at the top of 
the flower-bearing stalk. They will grow while still attached to 
the stem, but, of course, are detached for the purposes of prop- 

Corm. This type of stem is well exemplified in the crocus and 
gladiolus. It is a short, solid underground stem, differing from the 
bulb in the absence of scale leaves. The corm is usually flattened 
from top to bottom and bears a cluster of thick fibrous roots at the 
lower side and a tuft of leaves on the upper side. A number of 
small corms, called cormels, may develop on the mother corm. 
Both the large corm and the cormels may be used in propagation. 

Tuber. The best example of the tuber is the common Irish 
potato. It is a simple enlargement of the tip of a slender under- 
ground stem. The buds of the potato tuber usually occur in 
groups, each group being called an "eye." The potato plant may 
be propagated by planting the whole tuber, or by cutting it into 
pieces, each piece having one or more " eyes." 

Propagation by division. In this method of propagation, living 
vegetative organs are artificially broken or cut into sections, or 
pieces, and these placed under conditions suitable for growth. For 
example, as cited previously, the tuber may be divided into a 
number of pieces, each having one or more eyes (buds), and these 


may be employed to propagate the plant. Plants such as canna, 
iris, asparagus, rhubarb, and ferns which produce horizontal 
underground stems (rhizomes or rootstocks) may be readily propa- 
gated by dividing these into a number of sections, each of which 
bears several buds. Most of our perennial herbs may be propa- 
gated by division of the " crown/' The name " crown " usually 
applies to that part of a perennial plant which is just below or at 

f / Stolon 

Scale Leaf 

FIG. 110. -A strawberry plant showing propagation by means of "runners" or 
stolons. (From Holman and Robbins, in A Textbook of General Botany.) 

the ground surface, and which is essentially a clump of shortened 
stems bearing buds and roots. 

Exercise 101. Propagation by division. Propagate certain of the follow- 
ing plants by division: potato, canna, rhubarb, ferns, columbine, larkspur. 

Propagation by cuttings. In this method, some vegetative part 
of the plant is detached and placed under conditions favorable for 
growth. Cuttings may be made from stem, root, or leaf. Of 
course, the cutting must contain living tissue, have a certain 
amount of stored food for growth, and be capable of developing 
growing points. The growth of the missing organs from a cutting 
is regeneration. It is believed that any vegetative part of a plant 
(stem for example) which possesses active, growing cells has the 
power of regenerating the missing organs (roots, leaves, and 



flowers) if placed under proper environmental conditions. Failures 
are probably due to our inability to create satisfactory growth 
conditions. We have learned by experience that some plants can 
be propagated easily by stem cuttings, others by root cuttings, and 
a relatively few by leaf cuttings. We can not always tell by obser- 
vation of the plant whether or not these organs can be used to 
propagate its kind. We are still unable to offer a satisfactory ex- 



FIG. 111. Root and stem cuttings. From left to right: root cutting, hard- 
wood cutting, soft-wood cutting, and soft-wood cutting showing proper and 
improper age of stem from which cutting is to be made; the stem should snap in 
two when bent, as shown at the left of the leafy stem, rather than crush, as shown 
at the right. (Figure at right redrawn from Kains, in Plant Propagation.) 

planation why root cuttings of such plants as the blackberry and 
red raspberry will readily produce buds, whereas those of many 
other plants under the same conditions lack this power. 

Cuttings should be placed under conditions favorable for the 
rapid healing of the cut surface. When a stem is cut in such 
a way as to involve the growing layer or cambium, the cambium 
is stimulated to active growth and forms a mass of large, thin- 


walled cells known as callus. It is generally believed that callus 
formation must precede the development of roots, although it is 
known that roots do not arise directly from callus tissue. A new 
cambium arises by differentiation of inner callus cells, and as a 
rule it is from this cambium that the growing points of roots and 
stems originate. However, best results are usually obtained 
from callused cuttings. At any rate, the conditions which favor 
the development of callus also favor root formation. 
Stem cuttings. These may be made from: 

1. Mature or dormant growth, that is, from hard or ripened 
wood, or from 

2. Immature growth, that is, from soft wood or from herba- 
ceous stems which are vegetatively active. 

As a rule, in cuttings of mature growth, wood of one season's 
growth is employed, although sometimes wood two or more years 
old is used. Cuttings are usually made from 6 to 10 inches long, 
although they may be longer or shorter. Each cutting should 
possess at least one but preferably two or three buds. It is 
advisable to cut the lower end just below a bud. The cuttings 
are usually made in the fall or early winter. They are then 
stored in a place that is warm and moist enough to allow callus 
formation and some root growth but not warm enough to permit 
bud growth. Root growth will proceed at temperatures too low 
for bud growth. It is often the practice to bury the cuttings, in 
bundles, with the uppermost buds downward. Thus the cut end 
of each stem is near the soil surface, which, being warmed first 
in the spring, stimulates the development of roots. Cuttings 
should be set out in the spring before the buds open. 

It is possible to propagate most plants from immature growth 
although greater care is often required to get them to root than 
is necessary for hard-wood cuttings. Stems should be used that 
are neither too hard nor too soft, but break with a snap when 
bent. Such plants as begonias, coleus, geraniums, and chrysan- 
themums are propagated by green cuttings, or so-called slips. 
They are usually grown in hotbeds or greenhouses. It must be 
kept in mind in the growing of cuttings that the tissues above 
ground are subject to the loss of water for a period before there 
is root development and sufficient water absorption. This makes 



it necessary that the water-losing surface be reduced by removing 
most of the leaves, and that the cuttings be protected in some 
way from drying out. The small leaf surface that is left on the 
plant manufactures food that is used in the formation of roots. 

Root cuttings. Many plants can be propagated by means of 
root cuttings. The sections of the plants used in this case do 

FIG. 112. A stem cutting, showing the independence of root and callus for- 
mation. (From Bioletti, in Calif. Agr. Exp. Sta. Bulletin.) 

not possess buds. Buds develop during growth of the cutting. 
Root cuttings are planted shallowly, about J to f inch deep. 
Among plants that can be propagated by root cuttings may be 
mentioned the blackberry, red raspberry, horseradish, willows, 
poplars, osage orange, plums, cherries, and juneberry. Generally 



those plants which have a tendency to produce suckers from the 
roots can be propagated readily by means of root cuttings. 

Leaf cuttings. A number of plants, chiefly ones with thick, 
fleshy leaves, can be propagated by means of leaf cuttings. Among 
such are begonias, bryophyllum, gesneria, and gloxinias. The 
leaves may be cut into a number of pieces, each including a part 
of a main vein. Under proper growth conditions the ends of the 
veins callus, and roots and buds are developed. In bryophyllum, 
the whole leaf may be used, without cutting the veins; when this 
is placed in contact with moist sand, roots and buds are formed 
at the notches. (See Fig. 109.) 

Exercise 102. Cuttings. 

Each student should propagate a number of 
different plants by cuttings, including 
stem, root, and leaf cuttings. Suggested 
material is as follows: stem cuttings: 
chrysanthemum, geranium, carnation, 
coleus, begonia; root cuttings: horse- 
radish, willows, cherries, blackberry, 
dandelion; leaf cuttings: begonias, bryo- 
phyllum, gloxinias. 

FIG. 113. Propagation by layer- 
ing. A branch is bent down and 
partly buried. It is here shown 
wounded, which seems to hasten 
root development. (Redrawn from 
Kains, in Plant Propagation.) 

Propagation by layering. This 
is a method by which the plant 
is propagated on its own roots. 
It consists of bending over a por- 
tion of a branch and covering it 
with soil to keep it moist. Roots 
develop at the nodes that are 

covered, and buds develop at these rooted nodes. The section 
of the stem thus rooted may be severed from the parent plant 
and lead an independent life. In many plants, rooting of the 
buried stem may be hastened by injuring it. This appears to 
limit the movement of food and brings about its accumulation 
above the point of cutting from which roots develop. Plants 
which root readily at the nodes, when their branches come in 
contact with moist earth, are easily propagated by layering. 
As contrasted with cuttings, the layer is attached to the parent 
plant, and thus derives water and food from it until it becomes 
established. It is a more certain mode of propagation than that 



of cuttings. Among plants which are commonly layered are 
wisterias, honeysuckles, grape, passiflora, raspberry, and hy- 

Exercise 103. Layering. Propagate by layering some plant such as 
grape, raspberry, hydrangea, English ivy, or honeysuckle. 

Propagation by suckers. In the strict sense a sucker is a stem 
arising from an adventitious bud which develops on a root. The 
term is sometimes wrongly extended to include branch stems from 

FIG. 114. Cleft grafting. Left, making cleft; right, cleft being held open for 

inserting scion. (From photograph by Division of Pomology, College of 

Agriculture, University of California.) 

the base of the plant. Among plants which develop suckers may 
be named the silver-leaf poplar, black locust, blackberry, red 
raspberry, and plum. Propagation by suckers involves cutting 
off a portion of the root which bears the sucker, and trans- 

Propagation by stolons or runners. A stolon or runner is a 
stem that grows more or less horizontally along the surface of the 
ground. The best common example is the strawberry. This 
plant naturally propagates itself by means of runners. A runner 



sent out from the parent plant produces both roots and new 
shoots after which the runner may die, thus severing the daughter 
plant from the parent. The young plants which form at the 
rooting nodes of the runner may be cut off and set out. Stolons 
form roots naturally, but rooting may be hastened by covering 
them with soil. It will be readily observed that the layer is in 
reality an artificial stolon. (See Fig. 110.) 

Exercise 104. Suckering and propagation by runners. Observe in the 
field the suckers of such plants as mentioned in the foregoing paragraph. Cut 

FIG. 115. Cleft grafting. At right, FIG. 116. Steps in tongue or whip 
two views of the scion, and at left, the grafting, 

scions in position in the cleft of the 

off a portion of a root which bears a sucker, and transplant. Also observe in 
a strawberry bed how the plants naturally propagate themselves by runners. 

Propagation by grafting. This is a very old horticultural 
practice, and is in common use in propagating fruit trees. The 
fruit grower, in order that he may be certain as to the variety 



he will have, propagates them vegetatively, one of the chief ways 
being by grafting. The operation may also be carried on in order 
to change the size of the tree. For example, when pears are 
grafted on the more slowly growing roots of the quince, the stock 
retards the growth of the pear, and dwarfing results. Another 
object of grafting is to grow desirable varieties on disease-resistant 
roots, or on roots which will adapt the plant to various soil condi- 
tions. For example, the northern California black walnut is 
resistant to a soil fungus (mushroom) known as Armillaria, whereas 
the English walnut is not so resistant to the organism; this is one 

Fia. 117. A and B, side grafting; C and D, saddle grafting. (Redrawn from 


reason for the practice of grafting the English walnut on black 
walnut. The Northern Spy variety of apple is resistant to the 
woolly aphis which attacks the roots. This variety is used as a 
stock upon which to graft less-resistant kinds. The common 
peach is sometimes grafted on Davidiana root because of the 
latter's resistance to alkali. 



Three kinds of grafting are recognized: scion-grafting, bud- 
grafting, and approach-grafting. In scion-grafting, a stem called 
the scion, containing several buds, is attached to another rooted 
stem or root, called the stock, in such a way as to bring the growing 
layers (cambiums) of each together. After a time there is a union 
of the stock and scion. In budding, the scion is a single bud 
together with a small strip of bark. This is attached to the cut 
surface of the stock so as to bring the growing layers of stock 

FIG. 118. Method of bridge 
grafting in a girdled trunk. 
Scions are inserted under the 
bark, thus bringing about a 
bridging of the girdled zone. 
(Redrawn from Solotaroff, in 
Shade Trees in Towns and Cities.) 


FIG. 119. Steps in patch budding, often 
used with walnuts. 

and scion together. Approach-grafting, also sometimes known as 
inarching, consists in uniting two plants while they are still growing 
on their own roots. It is obvious that this is possible only between 
plants which are standing close together, or between different 
parts of the same plant. As a rule, approach grafting is executed 
by removing a piece of the bark, down to the cambium, of the 
two stems to be united, and binding these two cut surfaces to- 
gether. After the two have grown together, the scion is cut off 
below the union, and the stock above the union. Care should 



be taken to sever the scions gradually in order that growth will 
not be retarded. 

It is well known from experience that certain plants can be 
grafted upon one another, whereas others are united with diffi- 
culty. For example, peach can be grafted on the plum, but it can 
not be grafted on the apple. As a general rule, the more closely 
two plants are related botanically, the better are the chances of 

FIG. 120. Steps in shield budding. A, the T-cut in the stock; B, inserting 
bud; C, bud in place; D, bud tied; E, bud stick, showing bud ready for 

inserting in stock. 

a union by grafting. Plants belonging to different families can 
not be grafted. For example, it is impossible to graft the peach, 
which is a member of the rose family, upon the walnut, a member 
of the hickory family. Plants belonging to different genera 
within the same family may or may not unite readily. For exam- 
ple, in the rose family, the peach will unite well with the plum, but 



not so satisfactorily with the apricot. The tomato and the potato 
may be intergrafted; they represent different genera in the family 
Solanaceae. Plants belonging to different species within the 
same genus, as for example, crabapples and common apples, usually 
form a satisfactory graft union. And different varieties of the 

1^'IG. 121. A budded apricot tree. (.Photograph furnished by JJi vision ol 
Pomology, California College of Agriculture.) 

same species, as for example, the different varieties of peaches, may 
be grafted upon one another. In short, as a general rule plants 
belonging to varieties of the same species graft upon one another 
with greater certainty than plants of different species of the same 
genus, these in turn with greater certainty than those belonging to 



different genera of the same family; and plants belonging to 
different families will not unite at all by grafting. 

It should be kept in mind that, when two plants are united 
by grafting, each keeps its 
individual characters. If the 
crabapple (scion) is grafted on 
the common apple (stock), the 
branches which arise from the 
scion will bear crabapples, and 
those from the stock common 

In grafting, the stock may 
be the root, the crown, the 
main stem, the main branches, 
or the tips of the branches. 
There are many methods of 
joining the stock and scion, 
among which are the follow- 
ing: whip or tongue, cleft, 
bark, kerf, veneer, splice, 
saddle, and bridge. In all 
these methods, care is taken 
to bring the growing layers 
together and have them touch 
at as many points as possible. 

What are the purposes of 
grafting? Navel oranges are 

FIG. 122. Approach grafting, also 
known as inarching. The stock and 
scion are bound together, cuts being 
made in each (two stems at left), ex- 
posing the cambiums, to hasten 
union. (Redrawn from Kains, in Plant 

seedless; how is the variety propagated? 
tree girdled by gophers be saved? 

How can the life of a 

Suggested activity. Prepare a demonstration of different kinds of grafts 
using a large square of heavy cardboard to mount the prepared material. 

Problem 8. How did reproduction by means of sex in plants 


There is strong evidence that millions of years ago the only 
plant life of the world was slimy masses of one-celled organism, 
that the type of plant life or prehistoric waters and muddy shores 


was composed exclusively of sexless species. In our modern world, 
plants which probably most resemble these organisms are the 
blue-green algae, many of which grow in hot springs, withstanding 
temperatures which would kill most other modern plants. Flow- 
ering plants as we known them today, and pines and spruces, even 
ferns and mosses, formed no part of the scanty vegetation of those 
early-day landscapes. After millions of generations of asexually 
reproducing organisms, sexual reproduction came into existence. 
Scientists believe that it arose independently in several different 
kinds of plants and animals. 

Even today there are some plants in which sexual reproduction 
is so primitive that it seems that it can not have changed very 
much through all the ages since sex first appeared. One such plant 
is Ulothrix. This is an alga, whose fine green, almost microscopic 
threads grow anchored by a holdfast cell to sticks and stones in 
moving water. The Ulothrix cell is able to organize its proto- 
plasmic contents into several masses. These peculiar bodies break 
through the hard outer shell of the cell and swim away. The orig- 
inal cell is thus divided into several naked cells, each propelled by 
microscopic arms of protoplasm. Most of them swim and find 
favorable surfaces upon which to anchor and gow into new indi- 
vidual filaments. These cells are spores. They are asexual 
bodies in that they are capable of growing into plants, just as 
are those in moss capsules and those on the under sides of fern 

The spores of Ulothrix are not all alike. This is significant. 
They vary in size. The number of spores from a single cell may 
vary from one to thirty-two. If one spore is produced, this 
means that the entire protoplasmic body underwent no division, 
but was freed from the cell. If thirty-two spores are formed in a 
single cell, this means that the mother mass of protoplasm under- 
went five successive divisions. These spores are, of course, very 
small and contain very little stored food. Associated with these 
size differences are amazing differences in behavior. Large spores 
and those of intermediate size behave as asexual spores, that is, 
develop directly into a new individual Ulothrix plant. The small- 
est spores, however, are apparently incapable of developing into a 
new plant. If a drop of water containing these smallest bodies, 



swimming or squirming about, is examined with a high-power 
microscope, they are seen behaving in a most extraordinary fashion. 
They are coming together in pairs, a mating of the most primitive 
sort, and each pair is fusing to form a single cell. This new cell, 
formed by the union of two, now has the power of growing into 
a Ulothrix plant. Separated, the smallest spores perish; united 
they are strong enough to carry on the race. 

Fia. 123. From one to thirty-two naked cells may swim out of a singie Ulo- 
thrix cell. The larger, four-armed ones are able to make a go of it by them- 
selves, but the smaller ones must first undergo the simplest sexual process 
known. Several are shown engaged in mating. (From Robbins and Pearson, 
in Sex in the Plant World.) 

The mysteriously rejuvenating fusion of spore-like bodies 
which we call gametes in Ulothrix is the most primitive form of 
sex act known. In the development of the plant kingdom, when a 
spore first behaved as a gamete, sex originated. Thus sex origi- 
nated in the plant kingdom merely as a modification of an asexual 
method. It is significant that, once sexual reproduction appeared 
among primitive plants, it apparently gave them and their offspring 


which inherited sexuality such an advantage in the struggle to 
populate the world that they did not die out. On the contrary, 
sexually reproducing plants have gone a long way towards " in- 
heriting the earth." 

In a primitive plant like UlothriXj sex probably originated. 
In this plant the gametes are to all appearances similar. But in 
sex development in the plant kingdom, one gamete becomes a small, 
motile sperm, the other gamete becomes a large motionless egg. 
In all plants, in seaweeds, in mosses and liverworts, in ferns 
and lycopods, sperms have fundamental likenesses; they are 
small, motile, and have very little stored food. Eggs on the other 
hand are relatively large, inactive, and rich in stored food. The 
fusion of an egg with a sperm is the process of fertilization. Ordi- 
narily a sperm alone, or an egg alone, is incapable of developing 
into a new individual, but once an egg has united with a sperm the 
fertilized egg is able to develop into a new individual. 


1. Why do insects visit flowers? 

2. Of what use are the bright colors of flowers. 

3. Name five plants which produce more than one kind of flower. 

4. What is the difference between pollination and fertilization? 

5. What is meant by the expression: " The flower is not a sex organ? " 

6. It has been observed that apples, even in a good season, set no more than 
5 per cent of fruit. Explain. 

7. Why does a strawberry bed sometimes fail to fruit well, although 
flowers are produced in abundance? 

8. Are berries found on all sassafras trees? Explain. 

9. Describe the course of the pollen tube. 

10. Why do the seeds of fruit trees so seldom produce offspring true to the 

11. In a " double " rose, what is the origin of the extra petals? How does 
such a plant reproduce? 

12. Why are several varieties of pears often planted together in an orchard? 

13. Which are the most important hay-fever plants, those which are wind- 
pollinated or those which are insect-pollinated? 

14. What is the general relation between the height of a plant and its method 
of pollination? 

15. Flowers are frequently clustered. What is the advantage of this? 



Practical Plant Propagation, by ALFRED C. HOTTFS, published by A. T. 
De La Mare Company, New York, 1922. An exposition of the art and 
science of increasing plants as practiced by the nurseryman, florist, and 
gardener. 244 pages, 108 figures. 

Plant Propagation, by M. G. KAINS, published by Orange Judd Company, 
1920. A discussion of greenhouse and nursery practice. 322 pages, 213 

The Modern Nursery, by A. LAURIE and L. C. CHADWICK, published by 
the Macmillan Company, New York, 1931. A guide to plant propagation, 
culture, and handling. 494 pages, 107 illustrations. 

The Nursery Book, by L. H. BAILEY, published by the Macmillan Com- 
pany, New York, 1912. A complete guide to the multiplication of plants. 
365 pages, 152 illustrations. 

Sex in the Plant World, by W. W. ROBBINS and HELEN M. PEARSON. 
Published by D. Appleton-Century Company, 1933. 193 pages, 66 illustra- 
tions. The subject matter treated in this small volume may be judged from 
the following list of chapter headings: All Life from Life; Sex in Flowers; Sex in 
Ferns and Mosses; The Origin of Sex; Primitive Sex; Sex in the Scavengers 
and Parasites; Begetting without Sex; Parthenogenesis; The Discovery of 
Sex in Plants; Plant Courtships; Males, Females and Otherwise; Sex Traits; 
Sterility; How Plants Prevent Inbreeding; Hybrid Sterility; What Deter- 
mines Sex; Sex Chromosomes; The Secret of Heredity. 



As humans, our daily life, our health, our capacity for doing 
work, our happiness all are influenced by the conditions about 
us. The food we eat, the temperature and humidity of the room 
in which we work, the light by which we read, the people with 
whom we come in contact all have an effect upon our life. The 
sum total of our surroundings we call the environment. We can 
not escape it. Everything we experience with our senses see, 
feel, touch, hear, or smell is a part of our environment and has 
its influence upon us. 

So it is with plants. Their growth and development, and all 
the associated activities, are influenced by the various factors of 
the environment. The principal factors influencing plants are: 
(1) water, (2) heat, (3) light, (4) soil, (5) air, and (6) organisms, 
both plants and animals. The environment is a complex set of 
factors. All these factors are operating at once upon the plant. 
How the plant behaves, and what its structure is, are determined 
not by water alone, or light alone, or any other single factor alone, 
but by all of them exerting their influence at the same time. 

Problem 1, What is the importance of water to plant life? 

Life without water is impossible. Water is just as essential to 
plants as it is to animals. Water is the principal part of all living 
tissue. It constitutes about 80 to 90 per cent of the weight of 
protoplasm (living material). Protoplasm becomes less and less 
active as water is removed, until a state of dryness is reached 
which causes death. 

Water in the living cells maintains their turgor, which condi- 
tion is necessary for their proper functioning. A plant does not 



manufacture food, and grow, unless its tissues are well filled with 
water. A wilted plant is inactive. Moreover, the combined 
turgor of all cells maintains erectness in plants. The brittleness 
of young plants is partly due to water pressure within the cells. 

All materials which move from one part of the plant to another 
must be in a watery solution. All mineral salts must be dissolved 
in water before they can enter the plant through the root hairs. 
Also, oxygen and carbon dioxide can not enter or leave the plant 
cell except in solution. 

As we have seen, water is an essential raw material in the man- 
ufacture of food. That is, many of the organic compounds which 
occur in plants are formed by the chemical combination of water 
with certain inorganic compounds which are absorbed by the plant 
from the soil and air. But, as we shall see later, the amount of 
water actually found in a plant is a very small fraction of the total 
amount which is absorbed by the roots; a large portion passes 
through the plant and out through pores in the leaves to the atmos- 

Water is the chief limiting factor in the growth of most crops. 
The farmer, except in the most rainy sections of the country, is 
usually confronted at some time during the season with a shortage 
of water. This is particularly true in arid and semi-arid regions. 

Water is a most important factor determining the character 
of plants upon the earth's surface. The striking differences in the 
vegetation of the high mountains, the dry plains, the prairies, 
the eastern deciduous belt; in the character of the plant life of 
tropical rain forests, deserts, and tundra; in the vegetation of hill- 
side, brook-bank, gravel slope, bog, meadow, and open water are 
largely due to differences in the available water supply. 

Amount of water in plants. The amount of water in different 
plants varies widely. As a rule, plants growing under dry condi- 
tions contain less water in proportion to their total weight than 
plants growing in wet situations. However, many desert plants, 
such as cacti, may possess large amounts of stored waters. The 
percentage of water is usually greater in young, growing parts than 
in older portions of the same plant. Seeds and woody tissues con- 
tain less water than the leaves or young roots and stems. Tough- 
ness of tissue is usually associated with a low water content of the 


tissue. Succulency and tenderness of tissue usually signify a high 
water content. 

Exercise 105. Determination of water content of plant parts. Determine 
the water content of the following plant parts: (a) fruit of apple, (6) potato 
tuber, (c) grain of corn, (d) green leaves of any plant, (e) twigs of any woody 
plant, keeping bark and wood separate. (1) Weigh and record weight of 
vessels to be employed. (2) Cut or break up the material finely, place in 
container, weigh immediately, and record. (3) Place in constant-temperature 
dry-oven, and dry at a temperature of 90 C. until a constant weight has been 
attained. (4) Record dry weight. (5) Compute percentage of water. Why 
must care be taken not to subject the tissue to too high a temperature? What 
is the difference between dry matter and ash of a plant? 

The water problem of the plant. It is quite clear that one of the 
chief problems of a plant is to take in at least as much water as it 
gives off. Of course, the intake must exceed the outgo, for some 
of the water is used in the plant. However, absorption must 
at least equal transpiration (the water-losing process in plants) 
if the plant is to maintain life. The great dangers that confront 
most plants, particularly dry-land plants or those subject to dry 
periods, are too little absorption or too much transpiration. A 
plant dies when the rate of water loss exceeds the rate of water 
intake for any length of time. 

There is a stream of water through the plant from root hairs 
to leaves. Its rate of flow may be limited or restricted at two 
points: (1) at the point of entry (root hairs) ; and (2) at the point 
of exit (leaves). Plants withstand dryness in two general ways: 

(1) by increasing the amount of water taken in through the roots; 

(2) by limiting or retarding the amount of water lost from the 

The wilting of plants. The importance of water in the plant's 
life is well shown by the phenomenon of wilting. A plant tissue 
is made up of a mass of cells. When a cell is filled with water and 
various materials in solution, its walls are bulged outward and 
we say that the cell is turgid. If the water is removed from the 
cell, it becomes flaccid and wilts. If all the cells of a herbaceous 
plant are filled full of water, each cell wall being stretched out 
because of the pressure from within, the plant as a whole stands 
erect. Of course, it must be understood that most plants possess 


strengthening woody tissue and are not entirely dependent upon 
turgor to hold them in an erect position. But the leaves of all 
plants and the young stems of herbaceous plants are largely 
dependent upon the turgor of the cells for their rigidity. 

When a plant wilts, water is not being absorbed as rapidly as it 
is being used or lost. Its cells are not full. The freshness and 
crispness of lettuce, for example, are associated with turgidity of 
the leaves. Any condition by which transpiration can be checked, 
such as by a cool atmosphere laden with moisture, will prevent 
wilting to a large extent. 

Exercise 106. Loss of water from leaves. Place a handful of fresh, green 
leaves, free from water on the surface, under a bell jar. Set up at the same 
time and in the same way a bell jar which has no leaves. Place both in a window. 
After 30 or more minutes examine for presence of moisture on the inner sur- 
face of the jars. How do you account for what you see? Does the water come 
from the leaves? In what form does moisture escape from the leaves? Why 
do leaves become limp when they lose water? 

Exercise 107. Loss of water from a growing plant. Secure a small, 
vigorous, potted plant. Cover the entire pot with a piece of oiled paper or 
rubber cloth, fitting it up closely about the stem ot the plant and over the soil. 
This is to make sure that no water escapes from the walls of the pot or surface 
of soil. Now place a bell jar over the plant and set in a place suitable for 
growth. From where does the water come that collects on the inside of the jar? 

The transpiration stream. All exposed surfaces of the plant 
are losing water, but the leaves are the principal transpiring 
organs. The water passes off from plants in the form of water 

In many respects transpiration resembles evaporation, such as 
takes place from a free water surface. It is different from evapo- 
ration, however, in that it is controlled in part by the plant itself. 
For example, the rate of water loss, by transpiration, from a living 
plant is less than the water loss, by evaporation, from a dead plant. 
And the amount of water lost from a given area of leaf is less than 
that lost from an equal area of free water surface. Why? For 
example, it was found, for one plant, that a given area of free water 
surface lost about ten times as much water in an hour as an equal 
area of leaf surface. As we learned in Unit II, the living plant has 
numerous pores (stomata) in the epidermis of the leaves which 
open and close with changes in the conditions of the surroundings. 


Although water loss is a constant danger to the plant, the 
process plays an important role in that it maintains a stream of 
water (the transpiration stream) from the roots to the leaves, and 
throughout the entire plant body. There has been a general 
impression that, the greater the rate of transpiration, the greater 
the rate of intake of mineral salts by the root hairs. It has been 
shown by careful experiments, however, that an increase in the 
rate of transpiration does not proportionately increase the quan- 
tity of mineral nutrients absorbed. For example, if, by being 
placed in a dry atmosphere, a plant is caused to absorb and trans- 
pire water at double its former rate, the mineral salts absorbed are 
increased in amount, but are by no means doubled. 

How does water get out of the leaf? We have learned that 
surfaces of leaves are covered with an epidermis or skin composed 
of box-shaped cells, the outer walls of which are thicker than the 
inner and side walls, and often waxy in nature, such that they 
effectively prevent the loss of water through them. Here and 
there in the epidermal layers of the leaves are small openings or 
pores, known as stomata (Figs. 28, 29). Each pore or stoma is 
bordered or guarded by two cells, which differ from all other 
adjoining cells in their shape, in the possession of green coloring 
matter, and in their behavior. In most plants these two guard 
cells of each stoma or opening are capable of changing their 
shape, and by so doing bringing about the opening or closing 
of the pore. In a wilted plant the stomata are usually in a fairly 
closed condition, and hence the opportunity for water loss is 

There are usually more stomata on the under surface of a leaf 
than on the upper, and in some plants there are none at all on the 
upper surface. For example, in the apple leaf there are no stomata 
on the upper surface, whereas on the under surface there are 
approximately 161,000 per square inch. What is the advantage 
of this? In corn there are about 60,000 per square inch on the 
upper surface and 102,000 per square inch on the under surface. 
It has been computed that in a single corn plant of average size 
there are approximately 104,057,850 stomata in the epidermis or 
skin of its leaves. 

There is some slight loss of water through the waxy cuticle of 


a leaf, but most of the water loss is through the stomata. This is 
well demonstrated in the following exercise. 

Exercise 108. The loss of water from leaves is chiefly through stomata. 
Take three fresh green leaves of the common rubber plant (Ficus elastica) of 
conservatories. In this plant the stomata are confined to the lower surface. 
Smear vaseline on the upper surface of one leaf and on the lower surface of a 
second; and keep the third leaf free from vaseline on both surfaces. Hang 
them up by the leaf-stalks. Observe the results at various intervals of one, 
two, three, five, and ten days. Discuss. 

The amount of water lost by plants. The amount of water 
that is absorbed by plants, passed through their body, and out 
through the stomata to the air is enormous, and really much 
greater than most people realize. It was computed that a single 
corn plant during one growing season lost 54 gallons of water. A 
perfect stand of corn would be about 6000 plants per acre, so the 
total amount of water that would be evaporated from the leaves 
and sheaths of an acre of corn during the growing season would 
be 324,000 gallons, or 1296 tons. Disregarding all other losses of 
water from the soil, how many inches of rainfall would it require 
to supply the foregoing acre of corn? 

It has been calculated that an average-sized oak may have 
700,000 leaves; that about 244,695 pounds of water will pass from 
its surface in the five months, June to October. In a single year 
there will pass through the oak tree an amount of water equal to 
226 times its own weight. 

From which is the most water lost : an area covered with vege- 
tation, or one devoid of vegetation? From the water standpoint, 
why are weeds injurious in a field of crops? 

Exercise 109. Measuring the amount of water lost by leaves. Fill a jar 
with a measured amount of water. Cover the top with a rubber cloth. 
Secure a freshly cut healthy peach twig containing approximately 20 to 30 
leaves. Cut in the rubber cover a slit just large enough to allow the twig to 
enter. Allow the cut end of the stem to reach well down into the water. Ob- 
serve for a period of three or four days. If it is necessary to add more water, 
measure the amount. Determine for any time interval the amount of water 
lost from the jar. Does this represent that lost by transpiration through the 
leaves? Find out from the instructor a simple method for securing the com- 
bined area represented by all the leaves of the twig. Estimate the number of 
leaves on an average-sized peach tree, and from your computation make a 


rough estimation of the amount of water lost from the entire tree during a 
24-hour period. 

The water requirement of plants. Plants differ greatly in the 
total amount of water which is expended in producing a unit of 
dry matter, that is, in their water-requirement. Some plants, 
like millet, are economical; others, like alfalfa, are comparatively 
uneconomical. The water requirements of a number of plants 
have been determined experimentally. In the following table is 
given the water requirement of a number of plants under certain 
conditions as they existed at Akron, Colorado. For different cli- 
matic conditions these values will be somewhat different. 




Alfalfa, Grimm 659 

Rye, Spring 496 

Oats, Swedish Select 423 

Barley, Beardless 403 

Wheat, Kubanka 394 

Wheat, Kharkov 365 

Corn, China White 315 

Wheat, Turkey 364 

Sudan grass 359 

Milo, Dwarf 273 

Kaoliang, Brown 223 

Millet, German 248 

It will be observed from this table that alfalfa, for example, 
uses more than twice as much water to produce a unit of dry 
matter as does German millet. In general, a plant with a low 
water requirement is relatively drought-resistant. 

Problem 2. What is the relation of temperature to plant life? 

It is well known that the amount of heat a plant receives 
greatly influences its growth. The germination of seeds, the 
growth of roots, of stems, and of leaves, the opening of buds and 
flowers, and the development of seeds and fruits all are dependent 
upon certain temperature conditions. Every process of the plant, 
including such important functions as respiration, food manufac- 


ture, absorption, digestion, and reproduction, is influenced by the 
temperature. These different functions of the plant, and the 
growth of various organs, may have different temperature rela- 
tions. For example, absorption of water and salts from the soil 
will go on at lower temperatures than will the development of 
flower structures, and the seeds of a plant will usually germinate 
at a temperature lower than that which is necessary for the matur- 
ing of the seeds of that same plant. 

The temperature plays a great part in determining the distri- 
bution of plants over the surface of the earth. There is a decrease 
in the temperature as we go from the equator to the poles, and 
from low to high altitudes. We recognize the broad zones of vege- 
tation peculiar to the tropics, the subtropics, the temperate 
zones, and the arctic regions. Plants vary in their resistance to 
low and to high temperatures. And the yields and quality of 
orchard, garden, and field crops depend greatly upon the tem- 

Effect of temperature upon growth. When a plant grows, 
there is an increase in the number of cells and in the size of the 
cells. Two new cells result from the division of an old one. The 
two newly formed daughter cells, at first small, increase in size. 
Of course, the growth of the plant body as a whole is the result of 
the combined growth activity of the many cells which make up the 
body. Not all cells of a plant at any one time are growing at the 
same rate; in fact, they are not all exposed to the same environ- 
mental conditions. 

Every plant carries on its life processes between certain tem- 
perature limits. There is a certain low temperature below which, 
and a certain high temperature above which, a given plant can not 
grow. We call these temperatures the minimum and the maxi- 
mum, respectively. Somewhere between these two, there is a 
temperature at which the plant grows to the best advantage. 
This we call the optimum. These three important temperature 
points are called cardinal temperatures. 

The germination temperature influences the development of 
the adventitious roots in wheat. In this plant, the first whorl of 
adventitious roots forms much nearer the soil surface at high tem- 
peratures than it does at low temperatures. The plants in the 


former case are relatively weak. Seedlings of wheat, germinated 
at temperatures just above freezing, develop a root system two to 
three times as large as those grown at higher soil temperatures. 

Exercise 110. At what temperature do seeds lose their ability to germi- 
nate? (a) Secure seeds of wheat, corn, beans, radish, buckwheat, cherry 
(" stones "), etc. Place several hundred of each kind of seed in separate 
beakers or large test tubes and cover with water. Bring each to a temperature 
of 65 C. and retain at that point 15 minutes. Take out 25 of each kind of 
seed and place under conditions suitable for their germination. Raise the 
temperature of the remaining seeds to 80 C. and after 15 minutes arrange 
test for germination of each kind of seed. Raise the temperature to 95 C., 
and again after 15 minutes arrange test for germination. Raise the tempera- 
ture to boiling (100 C.) and repeat the procedure given above. (6) Repeat 
the above experiment, but instead of heating the seeds in water, subject them 
to a dry heat. A dry-oven may be employed, or a double-boiler may be 
improvised using two beakers of different size. Compare the results, (c) Soak 
lots of the above seeds in water for 1 or 2 hours. Expose lots of each, both 
dry and soaked, to different low temperatures. Different low temperatures 
may be obtained by preparing freezing mixtures of salt and ice. After exposure 
of seeds to the low temperatures for 1 hour, test their germination. Arrange 
the results of your tests in the form of a table and give your conclusions. 

Exercise 111. What is the effect of temperature on root growth? Place 
seeds of lettuce, radish, beans, corn, or other plants in germinators and germi- 
nate at room temperature. When the roots are about J4 inch long, place a 
number of sprouting seeds of each kind at different temperatures. Select 
certain sprouting seeds, marking their position in the germinating dish, and 
measure the length of the main root at intervals of 12 hours. Discuss results. 

Resistance of plants to low temperatures. It is well known 
that some plants will withstand a much lower temperature than 
others. For example, the date palm is usually injured by tem- 
peratures below 20 F., whereas most varieties of apples will 
endure temperatures much below zero, if the tissues are mature 
and in a dormant condition. It is also recognized that different 
tissues of the same plant vary in their resistance to low tempera- 
tures. In our common woody plants the tissue least resistant to 
freezing is the pith; the next least resistant is the sapwood; then 
the bark; and the most resistant, in well-matured and well-hard- 
ened stems, is the cambium. However, in actively growing stems, 
the cambium is not so resistant to freezing as other tissues. 

The more water plant tissues contain, the more readily are 
they killed by freezing. Active, growing tissues have more water 

FIG. 124. Heaters in a California orchard. In certain parts of the country 
late frosts threaten the blossoms of fruit trees. The United States Weather 
Bureau issues warnings of approaching low temperatures. Orchardists light 
the heaters, which may develop enough heat to prevent freezing of the tender 



than dormant tissues, and consequently are more easily frozen to 
death. Seeds that are well dried will stand much lower tempera- 
tures than seeds filled with moisture. For example, corn often 
suffers from freezing before the grain is quite dry. If the grain 
becomes thoroughly dried, it will withstand very low temperatures. 
Corn containing 10 to 14 per cent moisture may be stored with 
safety in bins exposed to temperatures considerably below F. 
A frozen grain of corn may have the appearance of being healthy, 
but the germ (embryo) may be killed or its vitality considerably 
reduced. It is very essential that corn seed be given a careful 
test for germination before planting. 

The maturing or " hardening " of plant tissues that takes place 
in late summer, autumn, or early winter seems to influence the 
resistance to winter temperatures. Well-matured or " hardened " 
tissues are more resistant than those not completely matured or 
hardened. For example, wood that has had late growth and gone 
into the winter incompletely matured is comparatively susceptible 
to winter injury. 

Gardeners commonly practice the process of " hardening off " 
their transplants. If a tomato plant is removed suddenly from a 
warm greenhouse in the spring to the garden out-of-doors, it has 
little resistance to low temperatures, and the death-point is rela- 
tively high. The usual procedure is to move the plants from the 
greenhouse to a cold frame where the temperature extremes are 
not so great as in the open; here the plants become adjusted to the 
lower temperatures and after a period may be planted with safety 
in the open. After the hardening-off process, the plant is able 
finally to withstand a lower temperature than if suddenly removed 
from a warm to a cool situation. Hardened plants differ from 
tender plants in having in the cells more soluble proteins and more 
water-imbibing substances. 


1. The minimum temperature for the growth of rye ranges from to 
4.8 C., whereas that of tobacco ranges from 10.5 to 15.6 C. What is the- 
relation of these temperatures to the distribution of these crops in the United 

2. Discuss the meaning of the expression: " An annual plant may be 


said to belong to no country in particular, because it completes its existence 
during the summer months." 

3. What is there about the habit of cereals which makes it possible to grow 
them in a wide range of climates? 

4. Why are partly open buds more easily frozen than dormant buds? 

5. What is the purpose of placing straw mulches over such low-growing 
plants as strawberries? 

6. A wet soil is usually a cool soil, whereas a dry soil is usually a warm soil. 

7. Cite various ways by which man controls the temperatures about plants. 

8. What are the chief factors which account for the difference in the type 
of vegetation as one proceeds from low to high altitudes, and from low to high 

9. Cite plants that will withstand very high temperatures. 

10. Why is it necessary that seed be thoroughly dry before it is stored for 
the winter? 

11. What are the dangers of an "early spring"? 

Problem 3. What is the relation of light to plant life? 

We have learned that light is the sole source of the energy 
without which no single living thing, whether plant or animal, 
could continue to exist. The enormous amount of energy which 
comes from the sun, and upon which the life activities of plants 
and animals depend, is absorbed by the 
green leaves and other green parts of 
plants. In the green plant this energy 
is converted into plant food. The de- 
velopment of a green color and the 
amount of food manufactured by a 
plant strictly depend upon the dura- 
tion and intensity of light. 

Light also affects the movement 
and position of plant organs. We are 
all familiar with the movement of the Fia 125. Rosette arrange- 
leaves of a house plant toward the light ment of the leaves of the 
when placed in a window. purple star-thistle, a weed. 

The size and form and structure of 

plants are also influenced by light. For example, plants growing 
in intense light often have a dwarf form, with very short stems, 
whereas plants in light of low intensity usually develop long stems. 


The movement and position of plant organs are influenced by 
light. As stated, the direction of light and its intensity have an 
effect upon the movement and position of plant organs. Leaves 
are especially sensitive to light. Many leaves assume a position 
which will expose a flat surface to the direct rays of light. 


FIG. 126. Leaf mosaic of ground ivy, show- 
ing a minimum of shading. 

Exercise 112. Light and 
leaf position. Observe in the 
open or greenhouse the rela- 
tion of shoots and leaves of 
any plant to light. Observe 
how the leaves are so arranged 
that there is little shading of 
one leaf by another. In fact, 
the leaves of ten form a mosaic. 
See Figs. 125 and 126. This is 
particularly evident in plants 
that form a rosette, or in 
climbing plants like ivy which 
adhere to a wall. Also ob- 
serve the position of the 
leaves of the wild lettuce, 
which on account of intense 
illumination take a position 
which exposes their edges to 
the sun during the hottest 
part of the day. (Fig. 127.) 

Exercise 113. Response 
of leaves to light. Grow 
seedlings of lettuce, peas, or 
beans in a pot which stands 
in a window, and observe the 
direction of growth of the 
plants. Discuss results. 

The size, form, and structure of plants are influenced by light. 

Intense light seems to have a stunting effect. This is shown by 
the low stature of alpine plants, although other factors, chiefly low 
temperatures and excessive transpiration, may play a part. Plants 
in the dark grow long and spindling, which tendency is noticeable, 
but less so, in the shade. Examine potato sprouts that have de- 
veloped in a dark cellar. Contrast with those which develop in the 
light. Why do house plants sometimes grow long and spindling? 



There is a marked difference between sun and shade plants 
of the same species. Trees in the open branch and spread pro- 
fusely, whereas the same species in the forest, where the light 
on all sides is cut off, grows taller and produces fewer side branches. 
How do you account for the splendid form of trees which grow 
in the open? The leaves of shade plants are thinner and broader 
than those of sun individuals. What is the advantage of this? 

FIG. 127. Wild lettuce, a compass plant. Left, as seen from south; right; 
as seen from west. Of what advantage is the habit here illustrated? 

As has been stated, in the total absence of light, plants do not 
develop chlorophyll. The leaves of shade plants, however, are 
often a deeper green than those of sun plants. This is due to 
the fact that the epidermal layer of shade plants is thinner than 
that of sun plants, and consequently the underlying green tissue 
of the shade leaf shows through. Moreover, sun leaves are 


frequently covered with hairs, scales, or a waxy coat which 
prevent the chlorophyll tissue beneath showing through. Such 
plants often have a grayish color. 

Duration of light. In a discussion of light and its influence 
upon plant growth, we must take into consideration its duration, 
its intensity, and its quality. In the arctic regions the summers 
have long days, but the light intensity is low; at the equator, the 
daylight period is shorter, but the light is very intense. The 
long daily period of sunlight at high latitudes, even though the 
light is of low intensity, makes possible the maturing of splendid 
grain and vegetable crops. It has been found that the accumula- 
tion of carbohydrates in plants, and their rate of growth, are in 

Fig. 128. Response to light. Two views of the same geranium plant. What 

was the effect of light on the length and position of the stem? How did the 

leaf petioles respond to light? The leaf blades took a position perpendicular 

to the incident rays of light. 

proportion to the number of hours the plant is exposed to the 
light, rather than to the intensity of light. 

Light intensity. Plants vary greatly as to the intensity of 
light .which they can withstand. Most plants do best in diffuse 
light; high light intensity is injurious in that it destroys the 
chlorophyll and thus retards the food-manufacturing process. 
Many plants, especially in the seedling stage, can not withstand 
direct sunlight for any considerable period. In full sunlight, 
the date palm leaf ceases to grow. Normal growth of this organ 
is made chiefly in the time between sunset and sunrise, but also 
to a slight extent in daylight when direct sunlight is cut off by 


It has been found by experiment that the Norway maple is 
capable of carrying on the manufacture of sugar when the light 
intensity is only ^V of the total daylight, whereas cherry is incap- 
able of performing this function if the intensity falls below of 
the total sunlight. In other words, Norway maple has the 
ability to endure shade. 

If a plant is grown under conditions in which light is insuffi- 
cient, it shows certain distinctive characters. For example, the 
color of the foliage is pale green and often sickly, the number of 
leaves is decreased, there is a scanty development of roots, the 
growth is more succulent, that is, less woody tissue is developed, 

FIG. 129. Morning (right), noon (center), and evening (left) positions of the 
same sunflower plant, showing response to the stimulus of light. 

the stems are long and spindling, and the plant may fail to bloom 
and produce fruit. If nursery trees are planted too close, so 
that there is not sufficient light, they tend to grow long and 
slender and to have a weak root system. It must be remembered 
that the vigor of a root system depends primarily upon the food 
brought to it from the leaves. A decrease in the leaf surface 
brought about by too close planting is equivalent to a decrease 
in the food-manufacturing surface. 

Sometimes advantage is taken of the response of the plant to 
decreased light brought on by close planting. For example, flax 
grown for fiber is planted in a closer stand than when grown for 
seed. The close stand induces the development of long, slender 


stems, which yield fiber of high quality. Also, in growing sor- 
ghums and corn for fodder, or for ensilage, where a large amount 
of succulent growth is desired, the plants are grown close together. 
Tobacco plants are often grown in the shade of tents, which condi- 
tion makes a larger and thinner leaf with less vascular tissue. 
The leaf is thus improved for wrapper purposes. 

FIG. 130. The evening-primrose opens its flowers in late afternoon or early 
evening. The illustration shows three stages in the opening of the flowers. 

In general, light of medium intensity promotes vegetative 
growth, whereas intense light favors the development of repro- 
ductive structures. In the northeastern states, where cloudy 
days are frequent during the growing season, there are splendid 
yields of potatoes, carrots, turnips, and other crops which are 
grown for their vegetative structures. On the other hand, the 



principal seed-producing regions are found in the western and 
middlewestern states where the percentage of sunshine during 
the year is high and the light intensity is relatively great. 

Blanching is .a process in which the plant is prevented from 
becoming green by growing it in the dark. To produce blanched 
(white) asparagus, for example, the plants are banked or ridged 
up with soil, so that the " spears " 
must make an additional growth 
of 4 to 10 inches before they 
come to light. The shoots that 
develop in the soil are, of course, 
whitish. The blanching of celery 
is accomplished by placing boards, 
paper, or earth about the stalks 
to exclude light. The heads of 
cauliflower are blanched by bring- 
ing the outer leaves up over the 
head and tying them, thus exclud- 
ing light. 

The intensity of light to 
which a plant is exposed may be 
increased by pruning and by a 
thin stand. One of the objects in 
pruning trees is to allow light to 
reach the center of the tree. 

The use of artificial light to 
supplement natural daylight and 
thus bring about the forcing of 
plants has been the object of much 
experimentation. Vegetables such as lettuce and radishes, 
kept under a strong arc light during a part of the night, become 
ready for the market from 10 to 14 days earlier than those exposed 
to normal light duration. Other kinds of artificial light have been 
used in forcing plants, among them the ordinary carbon incan- 
descent electric light, acetylene light, that of the Welsbach burner. 
Although artificial light is effective in forcing certain vegetables 
and flowers, its use is not usually attended with commercial gain, 
on account of the cost of the light. 

FIG. 131. Above, celery before 
blanching is green and not at all 
like the celery we see in the mar- 
ket. Below, celery in the process 
of blanching, soil is packed around 
the leafstalks to exclude the light. 
The portions of the leaves that 
are covered lose their chlorophyll 
and the later leaves develop with- 
out chlorophyll. 


Exercise 114. snaae and sun plants. Make a list of shade-demanding 
plants and of sun-demanding plants. Refer to the catalogs of nurserymen. 
Exercise 115. Extraction of the chlorophyll from leaves, and the effect of 
light on chlorophyll. Place the leaves from which chlorophyll is to be extracted 
in a flask and add water. Boil for a minute. Replace the water with 80 per 
cent alcohol and continue to heat on a water bath. Keep the alcohol vapors 
out of range of the flame. When the chlorophyll is extracted, filter the solution. 
Place a portion of the solution in the direct sunlight, and an equal portion in a 

dark cupboard. After 30 minutes com- 
pare the color of the two solutions, 
What is the influence of light upon chlo- 

The quality of light. The 

white light that shines upon the 
leaf is composed of a number of 
different rays, which vary in their 
effect upon plant growth. The 
visible spectrum, so beautifully 
shown in the rainbow, is com- 
posed of red, orange, yellow, green, 
blue, and violet light. Beyond 

FIG. 132. A cauliflower head. In the visible red are invisible rays 
the process of blanching cauli- known ag infra . red; and b ond 
flower, the broad, long leaves are . , . ., , . , , f 

tied about the head, excluding the the V1Slble Vlolet are ra ^ s of 
light and thus preventing the head 

from becoming discolored. 

invisible to the eye, known as 
ultra-violet. It has been demon- 
strated that the red rays of light 

are the most effective in sugar manufacture, and that the green, 
blue, and violet rays are the least useful of all in this process. 
Ultra-violet rays of light have an injurious effect upon plants. 

Problem 4. What is the relation of plants to the soil? 

The soil is the environment of roots. It is in the soil that 
plants are anchored in fact, frequently half or more of the 
ordinary plant body develops within the soil; it is from the soil 
that a plant absorbs water and mineral nutrients; most perennial 
plants store considerable quantities of reserve food in organs 
(roots or rootstocks) which are in the soil. 


The soil environment of a plant is very complex. It is more 
complex than the air. In its effects upon plants we must consider 
the soil from the three different standpoints; physical, chemical, 

The chief physical properties of a soil are texture and structure. 
Soil texture refers to the size of the particles which compose a 
soil mass. As a rule, we distinguish three general kinds of soils 
as to texture, namely, sandy soils (coarse), loam soils (medium), 
and clay soils (fine). 

Exercise 116. Kinds of soil. Secure three kinds of soil: sandy, loam, 
and clay. Shake an equal quantity of each in an equal amount of water, using 
the same kind of receptacle for each mixture. Set in a place where they will 
not be disturbed. Compare as to the time required for the soil particles to 
settle out, and for the liquid above the soil to become clear. What is your 
conclusion regarding the relative sizes of the particles in the different kinds of 

Soil structure refers to the arrangement or grouping of the soil 
particles. A sandy soil is usually of simple structure, in that 
the separate particles are much alike, and function separately. 
On the other hand, a clay soil may be very complex in its struc- 
ture, in that it may consist of soil granules of many different sizes, 
held together by glue-like colloidal material. Loam soils are 
usually regarded as having excellent structure. By this we mean 
that loam soils are not only porous, but they also hold moisture. 

As far as the plant is concerned, the soil is the source of most 
of the many chemical elements which enter into the plant's com- 
position. Throughout the ages, the rocks of the earth have slowly 
become fragmented and decomposed to form, along with decaying 
plant and animal material, the soil. Hence, most soils contain 
a mixture of both mineral matter and organic matter. We apply 
the term humus to the organic portion of the soil. Humus 
improves the physical condition of the soil, making of it a better 
medium for plant growth. We have learned that all substances 
which enter the roots must be in solution. Examination of the 
liquid portion of a soil shows that it consists almost entirely of 
water, carrying in solution many different kinds of mineral salts 
such as nitrates, phosphates, sulphates, etc. The mineral 
salts in the soil solution, together with carbon dioxide and 


water, are the raw materials from which the plant manufactures 

The soil as a medium is not wholly inert and lifeless. It is the 
home of countless micro-organisms, including bacteria, fungi, and 
protozoa. Bacteria and fungi, particularly, are indispensable in 
that they are responsible for the processes of decay of organic 
matter in the soil. Earthworms also play an important role in 
certain soils by aiding in maintaining its tilth. 

Water in the soil. All the water taken in by ordinary land 
plants is obtained from the soil and is absorbed by the roots. 
All substances which enter the plant must do so in solution, and 
the solvent is water. 

What are the chief conditions which influence the intake of 
water from a soil? These are as follows: (1) available water 
in the soil; (2) power of soil to deliver water; (3) extent of the 
root system; (4) temperature of the soil water; and (5) concen- 
tration of the soil solution. 

It is a well-known fact that, of the total amount of water in 
the soil, not all is available for plant growth. If we allow a plant 
to grow in a soil until it undergoes wilting, to the extent that it 
will not revive until water is added to the soil, we find that con- 
siderable water is still left in the soil. This is water that the 
plant can not get readily, and hence the plant shows distress. 
There is water in the soil, but the plant is unable to remove it 
and utilize it readily for growth. Hence, as far as the plant is 
concerned, the soil is dry. The percentage of water left in a soil 
at the time the plant undergoes permanent wilting is spoken of 
as permanent wilting percentage. This permanent wilting is not 
the same as temporary wilting which frequently takes place when 
the air is very dry. 

The amount of water available for growth varies with the 
soil. A plant can reduce the water content of a sandy soil to a 
lower point than it can reduce that of a clay soil. That is, when 
a plant growing in a sandy loam soil has used all the water it can 
for growth purposes, the percentage left in the soil is smaller than 
that left in a clay soil under similar conditions. 

For example, after a plant growing in a sandy loam soil has 
used all the water it can, without permanently wilting, there is 


left in that soil but 8.3 per cent water. On the other hand, the 
same plant growing in a clay loam wilts at a moisture content of 
13.6 per cent. Looking at this in another way, a sandy loam 
having 15 per cent total water would be much " wetter/ 7 as far 
as the plant is concerned, than a clay loam with 16 per cent total 
water. For the plant growing in a sandy loam with 15 per cent 
total water can reduce it to 7.8 or 9 per cent; the same plant 
growing in a clay loam would wilt when the water content was 
reduced to only 13.6 per cent. 

The above facts emphasize the need of knowing not only 
how much the total water in a soil is, but also how much of it is 
available for the growth of the plant. Most soils whose moisture 
content corresponds to the permanent wilting percentage are in a 
perceptibly dry condition and would be judged by anyone to be 
in need of water. 

But there are other considerations. For a long while it was 
thought that the greater drought resistance of one plant as com- 
pared with another was due to the greater ability of that plant to 
absorb water from the soil. It has been demonstrated that dif- 
ferent plants growing in a similar soil and under similar conditions 
have approximately the same permanent wilting percentage, in 
other words, that they reduce the percentage of water to about 
the same figures. The ability of a plant to resist drought appar- 
ently does not depend upon its power to extract water from a 

Power of soil to deliver water. If moisture is absorbed by 
root hairs from the adjacent soil particles at a very rapid rate, as 
on hot, dry days, it may not move from remote soil layers rapidly 
enough to supply that lost. It is clear that under this circum- 
stance the soil immediately surrounding the root hairs will become 
too dry to give up more moisture. Water moves from soil par- 
ticle to soil particle more rapidly in some soils than in others. 
The finer the soil, the slower are all water movements through it, 
but the extent of the movement may be greater. 

Extent of the root system. The character of the root systems 
of plants varies widely. There are root systems (1) that penetrate 
deeply in the soil; and (2) those that are confined to the surface 
layers. Some plants do not suffer from drought, because of their 


ability to send their roots into the deeper and moister layers of soil. 
Such a plant is alfalfa. If, on the other hand, the soil is shallow 
and the rainfall slight, the plants with a shallow root system may 
be somewhat more successful than deep-rooted sorts, on account 
of their ability to take advantage of the water that comes to the 
soil in the form of occasional light showers. It must be remem- 
bered that the depth of the root system is an inherited character 
of the plant, and is independent, to some extent at least, of external 
conditions. Root development, however, will not take place in a 
dry soil. Name five plants that have a shallow root system, and 
five that have a deep root system. 

Temperature of the soil water. The rate of absorption is low- 
ered by a decrease in the soil temperature. A plant may wilt in a 
soil saturated with water if the temperature of the soil sinks below 
a certain degree. In cold, dry climates winter killing may be the 
result of a cold soil, which slows up absorption, accompanied by a 
high transpiration rate. It is believed that in winter killing the 
plant is as frequently killed by direct drying as by actual 

Concentration of the soil solution. Water passes from the soil 
through the living membrane of the root-hair cells into the plant. 
This process of water intake goes on as long as the total concentra- 
tion of the cell sap is greater than the total concentration of the 
soil solution surrounding the root hairs. Other things being 
equal, the greater the concentration of the cell sap as compared 
with that of the soil solution, the more rapid the water intake. 
As the concentration of the soil solution approaches that of the 
cell sap, the rate of absorption slows down. Plants growing in 
an " alkali " soil are exposed to a soil solution of high concentra- 
tion. Hence absorption is retarded. There may be plenty of 
water present in the soil, but the plant gets it with difficulty, on 
account of the high concentration of the soil solution. Name five 
alkali plants. 

Likewise, bog plants are growing in a medium which retards 
water intake. This may be due sometimes to the high concentra- 
tion of bog waters, but more often to toxic substances in the soil, 
which hinder root development. Name five bog plants. 

The temperature of the soil. We just pointed out that the soil 


temperature influences the rate of absorption by the roots; absorp- 
tion is retarded or inhibited at low temperatures. Soil temperature 
also affects the growth of roots, the germination of seeds, and the 
various activities of soil organisms. 

The soil temperature is by no means always the same as the 
temperature of the air above it. It may be lower or higher than the 
air temperature. Numerous factors affect the temperature of a 
soil; chief of these are as follows: 

1. Air temperatures. Changes in the air temperature above 
a soil result in changes of the soil temperature. The fluctuations 
near the surface are almost parallel to those of the air, but at 
deeper layers the variations correspond to a lesser degree. The 
daily temperature change in bare, fallow soil extends to between 
12 and 24 inches from the surface. 

2. Exposure. By exposure is meant direction of slope. A 
north exposure, for example, faces north. The effect of exposure 
is much more marked at high altitudes than at low elevations. 
This greater effect is a direct result of the increased rate of radia- 
tion at high altitudes. The intensity of sunlight is distinctly 
affected by exposure and also by degree of slope. A given area of 
soil or plant surface that is at right angles to the direction of the 
rays of light will receive much more heat than one upon which the 
sun's rays fall obliquely, for under the latter condition the rays 
are spread out over a larger area than when they fall perpendicu- 
larly. If we assume the intensity of sunlight to be 100 when it 
strikes a surface at right angles, its intensity when striking that 
surface at an angle of 70 will be approximately 98.5; at an angle 
of 60, 96.5; and at an angle of 10, 33.4. Light intensity has its 
effect upon both air and surface temperatures, which indirectly 
affect the amount of moisture in the soil and the relative humidity 
over the soil. The differences between the native vegetation on 
adjacent north and south exposures is so conspicuous in the moun- 
tainous sections as to attract the attention of the most unobservant 
person. In a valley that trends east and west the slope exposed 
to the south has a much greater total effective heat during the 
year than the northerly exposure across the valley. The greater 
light intensity on the south exposure results in not only a warmer, 
but also a drier, habitat than that on the neighboring north 


exposure. A south exposure receives the greatest total heat during 
the day, the east the next greatest, then the west, and the north 
exposure least of all. On which exposure will plants bloom earliest 
in the spring? 

3. Living cover. A crop shades the ground and tends to pre- 
vent the soil from warming up. A bare soil warms up more quickly 
and cools off more rapidly than one covered with vegetation. 

4. Non-living cover (snow and mulch). It is well known that 
a snow covering prevents rapid changes in the temperature of the 
soil. The temperature of soil under snow is higher than that of 
soil unprotected. 

The temperature of a cultivated soil fluctuates to a less degree 
than that of an uncultivated soil. This is probably due to the 
poor heat-conducting power of the mulch formed on the surface 
of the cultivated soil. 

A non-living vegetative cover, such as a straw mulch, prevents 
rapid changes in the soil temperature. It has a cooling effect in 
the summer and a warming effect in the winter. In the winter 
the dead vegetative covering acts as a poor heat-conducting 
medium, which prevents a rapid loss of heat from the soil; and it 
tends to keep the cold air currents from coming in contact with 
the soil. It is common practice to place straw mulches over such 
low-growing plants as strawberries. Why? 

5. Moisture. A wet soil is usually a cool soil, whereas a dry 
soil is usually a warm one. Some of the heat absorbed by a soil 
is used in evaporating the water in it. A wet soil will absorb more 
heat than a dry one. Even a light shower will lower the tempera- 
ture of the surface soil to a considerable degree. Not only does it 
directly cool the soil by its addition, but, as stated, evaporation 
also lowers the temperature. 

6. Color. Dark soils absorb heat more readily than light- 
colored ones, and consequently heat up more rapidly. 

7. Physical nature of the soil. A coarse soil does not retain 
water readily, consequently it warms up rapidly. On the other 
hand, a fine-grained soil, like clay or loam, holds water well and as 
a result warms up slowly. It is customary to speak of coarse 
soils as " warm or early ," and of the fine-grained soils as " cold or 
iate." Compact soils conduct heat more rapidly than loose ones. 


This means that a compact soil will heat up quickly and cool off 
just as readily. 

8. Manures. The general effect of applying manures to a soil 
is to raise its temperature. In one experiment it was noted that 
20 tons of manure applied to an acre increased its soil tempera- 
ture 5 F. 

The air of the soil. The soil is porous. The pore spaces are 
filled with air and water. A moderately dry soil contains much 
air in its pores. A very wet soil contains less air than the same 
soil in a drier condition, for part of the space which would be occu- 
pied by water in the wet soil is occupied by air in the drier soil. 
In a water-soaked soil there is practically no air save that which 
is dissolved in water. It is well known that most ordinary plants 
can not thrive for long in a water-soaked soil, for there is an inade- 
quate supply of oxygen. 

The composition of the soil air is quite different from that of 
the ordinary atmosphere. Soil air is richer in carbon dioxide than 
that of the atmosphere. This is due to the fact that the roots 
and micro-organisms are absorbing oxygen and giving off carbon 

What is the role of air hi the soil? The living cells of the roots 
must have oxygen in order to respire. A scarcity of oxygen retards 
root-hair development and the absorption of water and of mineral 
salts. Seeds must have oxygen in order to germinate. Moreover, 
most of the bacteria and fungi and other living things of the soil 
require oxygen, and these have an indirect effect upon green 
plants growing in the soil. 

Mineral nutrients of the soil. The water of the soil carries in a 
dissolved form many different mineral salts, the so-called mineral 
nutrients. Many of these constitute raw materials used in the 
manufacture of food. They furnish to the plant such essential 
elements as nitrogen, potassium, phosphorus, sulphur, calcium, 
and iron. The soil solution also carries various gases, chiefly car- 
bon dioxide and oxygen, in addition to the mineral salts. 

The nutrient relations of plants are as different as are their 
bodily form and structure. There are plants such as blueberries 
which are intolerant of calcareous soils. We speak of plants 
which are " heavy feeders " and make great demands upon the 


soil. Tobacco is such a plant. Other plants are " light feeders " 
and do not draw heavily upon the chemicals in the soil. 

When plants are analyzed chemically we see readily that they 
have taken varying quantities of the different mineral nutrients 
from the soil, even when growing on the same soil. They make 
different demands upon the mineral elements in the soil. It must 
not be thought, however, that wheat, for example, growing in dif- 
ferent kinds of soils and under varied climatic conditions, would 
take the same amounts or relative proportions of the different ele- 
ments. We must consider averages based upon the chemical 
analyses of crop plants made in many laboratories. For example, 
a wheat crop yielding 30 bushels of grain and 1.6 tons of straw 
contains on the average 51.6 pounds of nitrogen, 8.6 pounds of 
phosphorus, 27.5 pounds of potassium, and 5 pounds of calcium. 
A 200-bushel yield of Irish potatoes removes, on the average, from 
the soil 42 pounds of nitrogen, 6.3 pounds of phosphorus, 53 pounds 
of potassium, and 55 pounds of calcium. A 15-ton crop of sugar 
beets takes from the soil 78 pounds of nitrogen, 10.5 pounds of 
phosphorus, 79.5 pounds of potassium, and 8 pounds of calcium. 
Thus we see that crops vary in their demands upon the different 
principal elements in the soil. Note in the figures above, for 
example, that a crop of wheat requires very much less potassium 
than a crop of potatoes or of sugar beets, but it requires more 
nitrogen than potatoes. 

A harvest of fruit from an orchard removes a certain amount 
of mineral elements, to which must be added those used in the 
making of leaves, stems, and roots. For example, the fruit only 
of a 100-barrel apple crop will remove from the soil on the average 
about 13.8 pounds of nitrogen, 2 pounds of phosphorus, 14.5 
pounds of potassium, and 1 pound of calcium. 

In the growth of plants for special purposes, man has attempted 
to find the nutrient conditions which will give him maximum 
production. He has learned that an abundance of water and of 
nitrates in proportion to potash makes for succulency in the plant, 
vegetative growth, and scant fruit production. On the other hand, 
if the supply of nitrogen is withheld to a degree and potash in the 
soil is relatively more available, fruit production is stimulated. 
It is well known that tomatoes on a soil excessively rich in nitrogen 


" go to vine " and produce little fruit. In certain wheat-growing 
sections it has been shown that an excess of available nitrogen 
over potash in the soil gives a flinty, hard grain; and that a 
starchy, mealy, and soft grain results if there is a lack of nitrogen 
and a relatively good supply of potash. Too much nitrogen, on 
the other hand, produces a tall plant, with a weak stem, which 
lodges easily. 

Fertile and infertile soils. In common understanding a " fer- 
tile soil " is one which will produce. A soil to be productive or 
fertile must have, of course, (1) the proper amount of water; 
(2) a supply of free oxygen; (3) a supply of available mineral ele- 
ments; (4) no harmful agents such as fungous diseases, weeds, 
insect pests, alkalies, acids, and toxins; (5) certain beneficial bac- 
teria and other fungi; and (6) a physical condition which is favor- 
able to seed germination and root development. 

In a more restricted sense, " fertility " or " infertility " has 
reference to the mineral nutrients, the so-called plant foods of the 

Man's control of the nutrient relation is largely concerned with 
making up a deficiency of some mineral nutrient brought about 
by the growth of plants. When a soil becomes infertile, that is, 
incapable of producing a normal yield, the infertility is usually 
due to a lack of either nitrogen, potassium, or phosphorus. These 
are the elements most commonly deficient in soils. The other 
3ssential elements are seldom lacking. Thus it is that most arti- 
ficial fertilizers contain one or more of these elements. Barnyard 
manure contains all the elements necessary to increase a soiPs pro- 
ductive power. Of course, it is true that manures of different 
farm animals differ in their chemical composition. Name several 
common commercial fertilizers. 

However, there is reason to believe that infertility of soil is not 
always due to a lack of essential mineral nutrients, although this 
is probably the most important cause. It appears that some soils 
with an abundance of mineral nutrients are non-productive because 
of toxic substances in them. There is evidence that, if a given 
crop is grown year after year on the same piece of soil, there accu- 
mulate in that soil toxic substances which are deleterious to the 
growth of that plant. Thus it would seem that one of the advan- 


tages of crop rotation is the counteraction of these toxic substances 
by the new crop. Further, it is believed that the value of adding 
manure in a case of this kind is in counteracting the. toxic sub- 
stances in the soil rather than in adding mineral nutrients which 
are deficient. 

In some instances, an unproductive soil may be due to organ- 
isms in the soil which attack the roots of the plant and cause 
disease. If one kind of crop is grown year after year on the same 
land, the soil fungi which prey upon that plant accumulate and the 
soil becomes non-productive, even though mineral nutrients are 

Living organisms in the soil. The upper layers of the soil 
teem with living organisms, chiefly bacteria. In addition to bac- 
teria there are various fungi, algae, protozoa, and worms. It is 
difficult to overemphasize the tremendous importance of bacteria 
in the economy of nature. The groups of soil bacteria particu- 
larly beneficial are those which bring about the decay of organic 
matter and those which fix nitrogen. At this point the student 
should review the discussion of soil organisms on pages 94-97, 
and throughout the following section reference should be made to 
Fig. 37. 

Bacteria in relation to soil fertility. Contrary to popular 
opinion, not all bacteria are harmful. In fact, many of them are 
absolutely essential in maintaining the life of the earth. Chief of 
these indispensable bacteria are those which bring about decay, 
breaking down complex organic substances such as proteins, fats, 
and carbohydrates into simpler substances that can be used again 
as raw materials in the manufacture of foods by green plants. 
Their presence and activity in the soil are necessary to maintain 
soil fertility. 

Ammonifying and nitrifying bacteria. Nitrogen is an essential 
element in plant growth. It is a constituent of the living material 
(protoplasm) itself. It is one of the principal components of both 
plant and animal proteins, and of many other chemical compounds 
in living bodies. It is well known that soil infertility is often due 
to a scarcity of available nitrogen, and that one of the principal 
ingredients of fertilizers is nitrogen in some form. 

Nitrogen occurs in the atmosphere as a gas. About 80 per 


cent of the air is nitrogen. However, green plants are not able 
to absorb the atmospheric nitrogen and use it in the building of 
foods. Although nitrogen gas, along with carbon dioxide and 
oxygen, passes through the pores (stomata) of the leaf, it is not 
utilized by the plant in the free, gaseous form. 

Nitrogen occurs in combination with many other chemical 
elements. For example, ammonia (NHs) is a combination of 
nitrogen and hydrogen, 1 part of nitrogen to 3 parts of hydrogen. 
Ammonia is a chemical compound of nitrogen and hydrogen. The 
nitrogen in this compound is not free, but is bound to hydrogen. 
In other words, the nitrogen is fixed. Another very common 
chemical compound is sodium nitrate or Chile saltpeter. This 
compound contains sodium, nitrogen, and oxygen in the propor- 
tion of 1 part of sodium, 1 part of nitrogen, and 3 parts of oxygen 
(NaNO 3 ). 

A great many mineral salts contain nitrogen, but of these, 
sodium nitrate, and its close relative, potassium nitrate, are the 
most important as sources of nitrogen for green plants. The nitro- 
gen in nitrates is spoken of as nitrate nitrogen. 

As has been stated, nitrogen is one of the most important ele- 
ments in plant and animal proteins. Manures are rich in nitro- 
gen, for they contain plant and animal products. But the nitro- 
gen in manures, or in any plant and animal refuse, is chiefly in a 
protein compound. It is protein nitrogen. It is significant that 
green plants can not use directly the nitrogen of proteins. It is 
necessary that the relatively complex protein compounds be 
broken down into simpler compounds of nitrogen, and that finally 
nitrates be formed. In other words, protein nitrogen must be 
changed to nitrate nitrogen. In all soils, under proper conditions, 
the nitrogen-containing compounds of manure are being changed 
to nitrates. This change is dependent upon the activity of three 
different kinds of bacteria. If these soil bacteria are not present, 
or if conditions are unsuitable for their growth and multiplication, 
manure does not decompose, and nitrate nitrogen is not formed. 

In the first place, the proteins of manure are decomposed 
through the activity of a group of bacteria known as ammonifying 
bacteria, and among the various decomposition products is ammo- 
nia, which of course contains nitrogen. The first step then is the 


change of protein nitrogen to ammonia nitrogen. Following this, 
another distinct group of bacteria changes ammonia nitrogen to 
nitrite nitrogen, and still another group of bacteria changes the 
nitrites to nitrates. The two groups of bacteria which change 
ammonia to nitrates are called nitrifying bacteria. In the three 
chemical changes brought about through the activity of soil bac- 
teria, the unavailable protein nitrogen has been changed to the 
available nitrate nitrogen. In this last form the nitrogen is 
readily absorbed by green plants, and utilized by them in the 
building of the proteins of their own bodies. 

It is seen that the soil teems with bacteria which are extremely 
beneficial and essential. It is clear that conditions in the soil 
must be such as to promote their growth and development. These 
organisms require a good supply of oxygen, a certain amount of 
water and warmth, and usually the presence of calcium or mag- 
nesium compounds. 

Nitrogen Fixation. It was stated in a preceding paragraph 
that green plants can not use free nitrogen gas of the air. The 
same is true of most plants and of all animals. However, a very 
few bacteria and other fungi are able to take free nitrogen and to 
build it into the nitrogenous compounds of their bodies. Such 
organisms have the power of nitrogen fixation. 

There are two principal groups of nitrogen-fixing organisms: 
(1) those which live on the roots of other plants, chiefly legumes, 
and (2) those which live without any association with the roots of 
higher plants. 

Legume bacteria cause the development of tubercles or nodules 
on roots. These tubercles or nodules vary considerably in size. 
Examination of a tubercle shows it to be composed of the swollen 
tissue of the host, in which are millions of the nitrogen-fixing bac- 
teria. Examine the roots of a number of different legumes for the 
presence of bacterial nodules. 

How the growing of legumes improves soils. It has been 
found that a clover plant, for example, secures about two-thirds of 
its nitrogen from the bacteria in the nodules, and one-third from 
the soil. Further, it is known that about two-thirds of the total 
nitrogen in the clover plant is in the tops (hay) and that the 
remainder is in the roots. Thus, it is seen that, when a crop of 


hay is taken from the land, there is removed an amount of nitrogen 
about equal to that coming from the air, and fixed by nodule bac- 
teria. The roots remain in the soil and in time decay, the nitro- 
gen they contain being returned to the soil. It will be clear, from 
the figures given above, that if a clover crop is to enrich the soil in 
nitrogen, it must either be plowed under, or fed to animals whose 
manure, which of course contains nitrogen, is returned to the soil. 
If the hay if sold off the farm, the growth of the legume has not 
enriched the soil in nitrogen. 

Denitrification. In addition to the ammonifying, nitrifying, 
and nitrogen-fixing bacteria of soils, still another group plays a 
part in influencing soil fertility. This is the denitrifying bacteria 
which change ammonia nitrogen to free atmospheric nitrogen. 
Such bacteria are undesirable from the soil fertility standpoint, for 
they take nitrogen from the soil. Denitrifying bacteria are most 
active in soils that are poorly drained and hence not well aerated, 
and in soils which contain large quantities of unfermented organic 

The nitrogen cycle in nature. As has been stated, nitrogen 
occurs in many different forms in nature: in the free gaseous form; 
as a part of inorganic compounds, such as ammonia, nitrites, and 
nitrates; and as a part of organic matter, either in the non-living 
or living form. In the processes of nature, nitrogen is constantly 
being changed from one form to another. Through the activity of 
denitrifying bacteria, and in electric discharges, nitrogen com- 
pounds are being broken down and nitrogen set free. The free 
nitrogen of the air in turn is taken by certain bacteria and changed 
into proteins and other compounds of plants containing nitrogen. 
The nitrogenous compounds of plants are changed to ammonia, 
the ammonia to nitrites, and the nitrites to nitrates. The nitrates 
are then used to rebuild plant proteins. Or plants are consumed 
by animals and the nitrogen of plants is used in the making of the 
nitrogenous compounds of animals. The organic nitrogenous sub- 
stances excreted by animals and the dead bodies of animals undergo 
decomposition, as a result of which nitrogenous compounds break 
down, liberating ammonia, which is in turn changed to nitrites, 
and nitrites in turn to nitrates. It is worthy of repetition to say 
that the processes of decomposition, of nitrification, of nitrogen 


fixation, and of denitrification are brought about through the 
action of bacteria. 


1. Why is there so little humus in the soils of arid and semi-arid regions? 

2. Compare the three common types of soil as to their ability to hold water. 

3. In canyons, gulches, or ravines which run east and west observe the 
differences in the plant life on north and south exposures. Explain. 

4. How does man artificially increase the temperature of the air about 

5. Describe the way to construct a hot bed; a cold frame. 

6. Give the physical characters of a warm or early soil; a cold or late soil. 

7. Mention three ways of raising the temperature of a cold or late soil. 

8. Why are crusted soils injurious to plants growing therein? 

9. Give five reasons for crop rotation. 

Problem 5. What is the relation of plants to the air? 

The entire plant is surrounded by the gases of the atmosphere. 
This includes the roots, which are surrounded by the air of the 
soil. The air that surrounds the plant supplies it with oxygen, 
necessary for respiration. All parts of the green plant absorb 
oxygen rather directly from the air which is about them; and in 
the presence of light, green plant tissue also absorbs carbon dioxide. 
Even the roots obtain oxygen from the soil air immediately sur- 
rounding them. 

In animals, the oxygen used in respiration, and the carbon 
dioxide eliminated in this process, are conveyed to and from the 
cells by the blood. In plants there is nothing corresponding to 
the blood stream. The sap of a plant does not carry appreciable 
quantities of these gases. In plants there is, however, an extensive 
system of air spaces between the cells, which communicate directly 
with the exterior through the stomata in the leaves, and through 
loose, open groups of cells (lenticels) in the bark. Consequently, 
the cells are surrounded by the gases of the atmosphere, and these 
can move inward and outward through the moist cell walls. Cer- 
tain types of cell walls will permit oxygen and carbon dioxide to 
diffuse through them, even when they are dry. 

In most crop plants the system of air spaces in the plant is not 
extensive or continuous enough to permit ready movement of air 



through the leaves and stems to the roots. Roots and root hairs 
absorb oxygen from the air which is present in the soil between the 
soil particles. In other words, the rapidly growing and active root 
hairs on a root which is several feet beneath the soil surface obtain 
oxygen for respiration chiefly from the soil air immediately sur- 
rounding them. There is no system of ventilating tubes which is 
adequate to convey sufficient oxygen from above ground to these 
subterranean structures except in aquatic and semi-aquatic plants. 
The normal growth and functioning of the root hairs, and conse- 
quently the absorption of water and mineral salts from the soil 

Fia. 133. Wind timber on the slopes of Long's Peak, Colorado. On wind- 
swept slopes of high mountains, near timber line, where the winds are prevail- 
ingly from one direction, the trees are often prostrate, and with twisted 
branches. (Photograph furnished by R. J. Pool.) 

and in fact the health of the entire plant body all depend upon 
an adequate supply of oxygen to the soil. It is known that the 
oxygen requirement of roots varies with the temperature of the 
soil. At a high soil temperature, the amount of oxygen necessary 
to give a normal rate of growth is greater than that required at a 
low soil temperature. Be that as it may, the fact remains that in 
all our treatments of the soil we should keep in mind the require- 
ment an ample and constant supply of oxygen in reach of the 
root hairs. The soil needs ventilation. It must be porous enough 


to permit the free movement of air through it. If it is crusted on 
top, owing to irrigation or rain, the free movement of air is inter- 
fered with, and the roots of plants are likely to suffer from a lack 
of oxygen. If the soil is extremely wet, the air supply is also 

Air Movement, We are familiar with the fact that clothes on 
the line dry much more quickly on a windy day than on a quiet 
day. The rate of evaporation, or loss of water vapor, is increased 
by wind movement. Wind is one of the several important factors 
which influence the loss of water vapor from the surfaces of plants. 
It does this by constantly removing from the surface of the plant 
the film or extremely thin layer of moist air which accumulates 
there, thus making way for the more free diffusion of water vapor 
outward from the air spaces within the leaf. 

Wind also influences the form of trees. Witness trees that 
grow at timber line on wind-swept mountain slopes, or along the 
sea coast. Here, as a result of winds which blow prevailingly in 
one direction, the trees often have most of the branches coming 
off the leeward side. This is due to the drying effect of the wind? 
which prevent buds from developing on the windward side. More- 
over, as a result of the continued mechanical strain, such trees fre- 
quently lean strongly leeward. 

Wind and Reproduction. Wind is of service in the reproduc- 
tion of plants in that it transports pollen and disperses seeds. 

Problem 6. What is the interrelation of plants and animals? 

There is an intimate interrelation between animal life and 
plant life. We have already called attention to the fact that the 
soil is traversed by many species of animals: earthworms, insect 
larvae, ants, etc. These animals are dependent upon plants for 
their food, and the animals in turn, especially earthworms, grind 
up plant remains into small pieces, mix their food with mineral 
particles, bury soil fragments in the soil, and by burrowing in the 
soil render it more porous. Thus, these animals render physical 
and chemical changes in the soil which are beneficial. 

One of the most intimate interrelations between animals and 
plants is seen in the insects which carry pollen. The flowers 


provide the insects with nectar and pollen; the insects carry the 
pollen and thus contribute to the plant's reproduction. Thus, 
the insect and the plants are mutually helpful. 

So completely dependent are some plants upon some one 
species of insect for pollination that they can not exist without 
that insect. Some time ago Australians decided that red clover 
would be a splendid crop plant for their soil and climate, so they 
imported some. But they could not get it to set any seed until 
they imported the red clover's special pollinator, the bumblebee. 

Fia. 134. The anthers of the meadow sage form a lever which the insect works 
himself. The anther lever is illustrated in the two upper flowers. After the 
anthers have withered the stigma grows down into a position where the insect 
must rub his back, covered with pollen from visits to younger sage flowers, 
against it, as shown in the lower left flower. (From Robbins and Pearson, 
in Sex in the Plant World.) 

Orchids are usually pollinated by moths. There is a famous 
cacc of an orchid, the Madagascar orchid, which produces its 
necLar in a spur nearly a foot long. On the strength of the exist- 
ence of this flower, a naturalist predicted that a moth would be 
discovered with a proboscis that long, although none was known 
at the time. Surely enough, such a moth was soon found fre- 
quenting the habitat of the Madagascar orchid. 


Very rarely, the insect seeks the flower, not to sip the nectar, 
but for a place to lay its egg. A remarkable instance occurs in the 

flowers of Yucca, a common desert 
plant. Reproduction in Yucca depends 
upon the assistance of the Pronuba moth. 
In turn the larvae of the moth gain their 
livelihood from the Yucca. The female 
moth with the aid of her special tentacles 
collects from a number of flowers a 
mass of pollen. Then, while still cling- 
ing to her cargo, she pierces the ovary 
of a Yucca flower with her long egg- 
depositor and lays an egg within the 
ovary tissue. This duty having been 
performed, she proceeds promptly to the 
stigma of the flower and presses the 
pollen ball into the stigma. This process 
of collecting pollen, of depositing an 
egg in the ovary of a flower, and of 
pounding the pollen into its stigma is 
repeated time and time again by the 
insect. Yucca ovules are fertilized; thus 
does the plant profit. Moth larvae 
hatch within the ovary and live upon 
some of the developing seeds; thus does 
the insect profit. It is no exaggeration 
to say that the very existence of Yucca 
plants depends upon the strange habits 
of Pronuba moths, and likewise the con- 
tinuity of the moth upon this earth is 
dependent upon the Yucca. 

The wasp "psen" which Aristotle saw 
fly out of a fig thousands of years ago, 
was born and raised there, and was on 
its way to crawl into another fig to lay 
its eggs and die, like all the females of 
its kind ever since that day and long 
before. A fig is a peculiar sort of flower- 

FIQ. 135. The Pronuba 
moth "hand pollinates" 
the Yucca flower. Each 
time she lays an egg in the 
yucca ovary, she makes 
sure that yucca seeds will 
be growing to feed her 
larvae when they hatch. 
In the upper flower a moth 
is collecting pollen; in the 
lower flower a moth is lay- 
ing an egg; in the central 
flower a moth is placing a 
ball of pollen on the stig- 
ma. The pod shows holes 
through which the Pro- 
nuba offspring have es- 
caped. (From Robbins 
and Pearson, in Sex in the 
Plant World.) 



bearing stem which has grown long and fleshy at the periph- 
ery until it completely encloses the whole cluster of very 
small staminate and pistillate flowers. The Blastophaga wasp 
goes through its entire life history from grub to adult within 
the fig. The males never see the outside world; they fertilize the 
females and die. But the females crawl out of the fig. By the 
time they get out they are well dusted with pollen from the male 
fig flowers they have had to crawl over. Now they enter other 
figs in quest of female flowers in which to lay their eggs. The 
wasps do not know it, but they can not get their eggs into the 
ovaries of ordinary female flowers. However, the fig also bears 

FIG. 136. Pollination of the fig. A, median lengthwise section of a fig show- 
ing fertile female flowers; note the female fig wasp near the opening, also 
another one inside. B, similar section of a fig showing gall flowers. (From 
Robbins, in Botany of Crop Plants.) 

a peculiar deformed type of female flower in which they can lay 
their eggs, and they finally do. But in the meantime, while they 
have been trying out the good female flowers, they have scattered 
pollen plentifully over the stigmas, and the fig seeds will set and 
the fig race be perpetuated. 

The seeds and fruits of many plants are a source of food for 
animals. Unwittingly, often the seeds are scattered by animals, 
and thus does the plant profit. Many birds eat fleshy fruits, the 
seeds either being regurgitated or passing through their alimentary 
tracts uninjured. Such seeds are quite likely to be deposited 


under conditions suitable to their germination. The seeds or 
fruits of some plants are provided with devices which enable them 
to adhere to the hairs of animals and thus be disseminated to new 
soil areas. As examples we cite the beards of certain grasses, 
the spines of the sand-bur, and the hooks or barbs of the cocklebur. 
Squirrels carry away and hide nuts, some of which may find 
favorable conditions for their germination. 

A few plants have means for securing animal food. The 
leaves of Venus' fly-trap, sundew, pitcher plants, and bladderwort 
are so constructed as to capture insects, which are finally used 
as food by the plant. 

The character and distribution of plants in the world have 
been profoundly influenced by man. He has domesticated many 
wild plants; he has hybridized them and carried them to all parts 
of the world, far from their original homes. He has removed 
forests, plowed the plains and prairies, and introduced grazing 
animals, thus greatly modifying the natural plant covering. 


Productive Soils, by W. W. WEIR, published by J. B. Lippincott Company, 
Philadelphia, 1920. 395 pages, 234 illustrations. Definite and practical 
information concerning soils and profitable crop production. It treats of soil 
and its origin, soils from a chemical point of view, soil and plant relations, crop 
production, factors determining soil fertility and principles of soil fertility and 
soil management. 



We speak of the surroundings of plants as their environment. 
The study of plants in relation to the environment in which they 
are living is known as plant ecology. Different factors of the 
environment which affect plants are water, light, air, soil, tempera- 
ture, and living things, both plants and animals. Any condition 
in the surroundings which causes a change in a plant is known 
as a stimulus. The effect that is caused by a stimulus is a 
response. Only living things can be affected by a stimulus. 
There can be no response in a lifeless object. 

Most seed plants have both a soil environment and an air 
environment. That is, they have their " feet " in the soil, and 
their " heads " in the air. The roots grow in the soil and they 
are related by structure in a way that fits them to the conditions 
of the soil environment. The factors of the soil environment are 
water, temperature, soil salts in solution, solid mineral substances, 
soil air, and soil organisms. In a way, we may also consider 
gravity as a factor of the soil environment. It differs from the 
other factors, however, in that it is nearly constant all over the 
surface of the earth, while the others are extremely variable. 
The roots are fitted to absorb water and salts from the soil solution 
and to anchor the plant in the solid earth materials. 

There may be substances in the soil which are injurious to 
plants. Such substances as acids and alkali may be present in 
the soil solution in concentrations which make it impossible for 
plant processes, as absorption, to go on in a normal way. Plant 
species differ in their ability to maintain life and grow under 
severe conditions. When a plant is resistant to an unfavorable 
factor of the environment, we say the plant is tolerant of that 



particular condition. Thus we have acid-tolerant plants, drought- 
tolerant plants, and alkali-tolerant plants. 

Plant breeders have produced, by hybridization and selection, 
varieties of cultivated plants which are tolerant of conditions 
under which other varieties of the same genus would fail. Most 
varieties of alfalfa winter-kill in the severe winters of our northern 
states. Grimm alfalfa, developed from a cross between common 
alfalfa and yellow-flowered alfalfa, is tolerant of the low tem- 
peratures of the northern winters and is grown successfully in these 

As we look about and see plants growing in nature, we note 
that some species of plants are found flourishing under conditions 
of extreme dryness, as lichens and mosses on the face of a rock 
cliff. We see others wholly or partially submersed in the water 
of ponds and lakes, as Elodea and water lily. Other plants with 
peculiar structural features can survive the conditions of the 
extremely dry habitat (we speak of the place in which a plant is 
growing as its habitat); likewise, only plants fitted to such an 
environment could live in water. In each type of situation we 
are apt to find a number of species living together, each fitted, 
by structure, to the particular habitat. Plants which are not 
tolerant of the conditions of a habitat are not found in the group. 
Thus we find plants of the same species and those of similar 
species of plants regularly living together in plant societies as, 
for example, the pond society and the desert society. 

When we study the fitness of plants in any situation, we are 
impressed with the fact that the two roles of plants are nutrition 
and reproduction. A plant must be able to provide food for itself, 
but it must also provide for the future of the species by some 
means of reproduction. Many plants have the ability to repro- 
duce rapidly by vegetative means, as the strawberry by stolons 
or runners, the horseradish by roots, the quack grass by rhizomes, 
and the Jerusalem artichoke by tubers. Seed plants, in general, 
must be fitted by structure for the processes of pollination, for- 
mation of gametes, fertilization, and subsequent development of 
the seed, in which is tucked away the young plant, together with 
a food supply sufficient to give it the necessary start in life when 
planted. Various structures in connection with fruits, as wings, 


hooks, and barbs, provide aid in dispersal of seeds by wind or 

Problem 1. To what kinds of stimuli do plants respond? 

When we consider the relation of plants to their surroundings 
we must realize that they are living things, their active parts 
being composed of the living material which we call protoplasm. 
We can study the properties of protoplasm, but we know very 
little about the reasons for its power to move, to grow, or to be 
affected by stimuli. Scientists have learned that living things 
behave as they do because of two major factors, both depending 
upon the nature of the living stuff, protoplasm. First, they 
inherit certain characteristics from their parents; and second, 
they are influenced by their surroundings. 

Tomato plants always have the same general characteristics 
of stem, leaf, and fruit. We can always recognize the plant as 
tomato. However, when plants of tomato grown in the sun are 
compared with plants grown in the shade, marked differences 
are found. The shade plants are apt to be tall, have slender 
stems and thin leaves, and bear little or no fruit; sun plants of 
tomato on the same type of soil will be strong and sturdy and 
set an abundance of fine fruit. The characteristics which enable 
one to recognize the plants as tomato are inherited; those which 
vary with change in habitat are the result of reactions to conditions 
in the surroundings. 

How do green plants respond to light? If you have tried to 
grow a plant by a window of your room, you have noted that the 
stem gradually became curved toward the light. You probably 
noted also that the petioles of the leaves twisted in such a manner 
as to bring the leaf into a plane transverse to the direction of the 
rays of light. A maple tree growing in an open field where light 
comes from all sides develops a low, symmetrical, and dense top 
with branches extending from the trunk almost to the base, 
whereas a tree of the same species in the thick forest grows a 
tall and straight stem having branches only at the top where light 
penetrates. See Figs. 197, 152. 

Exercise 117. What effect does light have upon the form of the plant? 
Grow in darkness a pot of bean plants for two weeks or longer. Compare 


general shape of plants, size of stem, size of leaves, color, etc., with the same 
features of plants growing in normal light for the same length of time. Answer 
the question of the exercise by means of diagrams. 

Exercise 118. What effect does gravity have upon the primary root of 
the plant? Plant corn grains which have been soaked over night, with tip 
of grain down, slightly below the surface in moist sawdust or sand. Remove 
the grains when the root is )4 inch in length and transfer to a moist chamber 
prepared as follows: Fasten four cork stoppers to the bottom of a dish by 
means of melted paraffin. Pack into the bottom of the dish about the stoppers 
absorbent material, such as moss, or bits of newspaper, filling up and leveling 
off after wetting the material until it is even with the tops of the attached 
corks. Place sheets of blotting paper over all, cutting to extend slightly up 
along the edges of the dish. Fasten four sprouted grains of corn through the 
blotting paper into the corks by means of pins so that the roots point in dif- 
ferent directions. Cut a piece of glass to fit as a cover and fasten in place with 
adhesive tape. Support on edge in a shallow dish holding water to supply 
moisture which will rise in the absorbent material by capillarity. Make a 
drawing of the moist chamber as set up, showing definitely the position of 
the roots at first. In what direction did gravity pull on the roots while the 
seeds were germinating in the sawdust? In how many different directions 
is gravity pulling on the four roots now? In so far as the seedlings are con- 
cerned, you have changed the direction of the pull of gravity. Examine after 
a day to determine the direction taken by the root tips in the meantime. 
Make a drawing showing the seedlings as they appear now and compare 
with the drawing made when the moist chamber was set up. What has 
been the response to gravity in this experiment? 

Exercise 119. What effect has gravity upon the stem of the plant? Sup- 
port a vigorously growing potted plant in a horizontal position in a dark room 
and examine after 24 hours. What change has taken place in the stems of the 
plant? By changing the position of the stem you have changed the direction in 
which the pull of gravity is applied to it. What is the response in this case? 
What is the stimulus? Compare the stimulus of Exercise 1 18 with the stimulus 
in this experiment. Compare the response of Exercise 118 with the response 
in this experiment. In a single connected statement, answer the question at 
the beginning of this exercise. 

It is evident from the results of our experiments that the root and the stem 
are affected by gravity in quite different ways. In some way which has not 
been explained, gravity causes the taproot of a plant to grow downward and 
the main stem to grow upward. These reactions are beneficial to the plant as 
they take the root system down into the soil where there is water together with 
nutrient soil salts. They take the shoot up into the air where the leaves are 
exposed to the necessary sunlight and where the flowers and fruits are in a 
position which favors the processes of pollination and seed dispersal. Give 
two reasons why the main stem of a tree usually grows upward. Give two 
reasons why the branches of a tree usually grow outward from the main stem. 


Tropisms. Curvatures in plant organs, such as those caused 
by light and gravity, are known as tropisms, and the organ of the 
plant which is affected is said to be tropic. Tropisms due to the 
stimulus of light are phototropisms, and those due to gravity are 
geo tropisms. The primary root is positively geotropic and nega- 
tively phototropic, while the main stem is negatively geotropic 
and positively phototropic. Other responses of plants are those 
caused by the stimuli of chemicals, chemotropism; those caused 
by water, hydrotropism; and those caused by contact, thigmotrop- 

FIG. 137. Explain what you see in this picture. What response determined 
the behavior of the roots of the tree? 

isin. Roots grow towards available soil salts and water and 
around solid objects with which they come in contact. It is 
sometimes necessary to cut down willow and poplar trees which 
are near tile drains because of the tendency of the roots of those 
trees to enter the drains through the cracks between the separate 
tiles and fill up the tiles with masses of fibrous roots, completely 
stopping the flow of water. It is a well-known fact that an 


occasional thorough soaking of a lawn will produce a deeper root 
system of the grass than frequent light sprinkling. How can you 
explain these facts in terms of the foregoing discussion? 

It should be remembered that the whole plant, above and 
below the surface of the soil, is in a complex environment, subject 
to the application of a number of forces. The form and nature 
of the plant, including root, main stem, branches, and leaves, is 
governed by all these forces acting at the same time. On the side 
of a cliff the taproot of a young seedling may take a horizontal 
direction into a crevice which holds moisture, reaction to moisture 
being greater in this case than reaction to gravity. Secondary 
roots are not affected by gravity in the same way as primary roots. 
Their course extends out from the main root approximately 
perpendicular to the line of pull of gravity. Although the reasons 
for this are not understood, it is recognized as a definite advantage 
to the plant as it enables the root system to reach all parts of the 
soil within the radius represented by the length of the longest 
lateral roots. The farmer, in cultivating his growing corn, digs 
the soil deeply at first to prepare loose soil for the spreading second- 
ary roots; then later he uses shallow cultivation to avoid injury 
to the lateral roots while he prevents the growth of weeds and 
provides a mulch of fine soil to hold the moisture. 

The leaves of most plants are dia-phototropic. By this we 
mean that the petiole twists in such a manner as to place the blade 
in a plane at right angles to the incident light rays. At the same 
time the leaves are placed in such a position with reference to 
each other that a minimum of shading is secured, so that in looking 
down upon the plant one sees a mosaic of leaves. The mosaic is so 
complete in some trees, as the Norway maple, that scarcely a 
speck of sunlight can be seen in the shade of the tree. The 
mosaic is well shown also in the rosette of mullein, dandelion, or 
evening primrose. It is also shown in an interesting way by 
Boston ivy on a wall where the light reaches the plant from one 
side only. Mosaics are shown in Figs. 125, 126. 

Give one case in which an animal reacts to light and two cases 
in which a plant reacts to light. Compare the reaction of an 
animal to light with the way a plant reacts to the same stimulus. 
Why does a plant react differently from an animal? 


Problem 2. How are plants related by structure to the water supply? 

With respect to water there are many different types of plant 
habitats in nature. They range from those types in which there 
is an abundance of water, as in ponds and marshes, to those in 
which there is a scarcity of water, as on the bark of trees, on the 
surface of rocks, or in desert sands. In chese different situations 
we find plants related to the water supply in different ways 
according to the conditions in the individual habitat. Plants of 
the same species are not found in all these situations. A plant 
species has inherited qualities which fit it, in general, to live in a 
certain type of habitat. There is a certain condition, as to water 
supply, in which the plants of a species are most likely to succeed. 
There is, however, frequently a rather wide range of habitats in 
which the plants of the same species will grow. Most plants have 
the ability to react immediately to changed conditions in a way 
that is an advantage to the plant. Plants growing where water is 
scarce are usually structurally different from plants of the same 
species growing where the water supply is sufficient. Likewise, 
plants growing in bright sunlight are different from plants growing 
in the shade. It is a well-known fact that lettuce grown early in 
the season while the water supply is ample and the days are 
neither too hot nor too long is apt to be crisp and delicious, while 
that from a later sowing which reaches development during the 
hot, dry weather of mid-summer will be tough and bitter. 

How are plants fitted for absorption? As we have seen, the 
younger parts of the roots are covered with a fuzzy growth of root 
hairs. These make possible rapid absorption of water by increas- 
ing the area of suitable absorbing surface. Water roots are nor- 
mally without root hairs, but most land plants have the root hairs 
exceptionally well developed. 

The roots of tropical orchids are peculiarly fitted for water 
absorption. These plants are known as epiphytes, since they live 
upon other plants from which they derive no nutrient materials. 
Their roots do not touch soil, and the whole plant is suspended in 
an air habitat. The air surrounding the roots is usually quite 
moist, but this has only the indirect effect upon the water supply 
of reducing the evaporation from the plant. The only water avail- 



able to epiphytes is that which wets the absorbing surface in the 
form of rain or dew. A spongy outer layer of roots of orchids, 

known as the velamen, takes up 
the water by capillarity in the 
same way that blotting paper takes 
up water. From the velamen the 
water passes inward from cell to 
cell by diffusion until it enters the 
conductive system of the root. 

The seed plant, Spanish moss 
(Tillandsid) , which hangs from 
the branches of trees in great 
festoons in the southern states, 
has no roots. Absorption is ac- 
complished by special structures 
on the leaves which take in water 
that wets the plants as rain or dew. 
Carnivorous plants. The word 
carnivorous comes from the two 
Latin words, carnis (flesh), and 
vorare (to devour). We are not 
accustomed to think of anything 
but animals as using flesh for food. 
A few plants, however, have the 
power of digesting and absorbing 
animal substances. Examples of 
this group of plants are, the 
pitcher plants, sundew, and 
Venus' fly-trap, all living in 
swamps and bogs. Plants living 
in these situations have poorly 
developed root systems, and as 
a result water absorption is re- 
stricted. This condition also 
hinders the absorption of nitrogen 
salts in sufficient amount to supply 
the needs of the plant for raw ma- 
terials. Besides, there is very 

FIG. 138. Drawings made from 
cross-sections of three types of 
leaves. A shows the internal struc- 
ture of a leaf of a plant immersed 
in water. Account for the presence 
of thin epidermis and large air 
spaces, and the absence of sto- 
mata. Note in B the heavy cuti- 
cle, thick epidermis, compact in- 
ternal structure, and location of 
the stomata in depression and pro- 
tection by hairs. Explain how 
these features fit the plant to living 
under desert conditions. C shows 
the section of a leaf grown under 
conditions of moisture intermedi- 
ate between those of A and B. 
Explain presence of the thin epi- 
dermis, moderately compact inter- 
nal structure, and exposed stoma. 


little mineral material in the waters of the peat bog, and this 
places an additional difficulty in the way of the plant's securing 
mineral salts. In some way, which is not well understood, these 
plants, in the course of their evolution, have developed the car- 
nivorous habit. 

In pitcher plants the modified leaves form pitcher-shaped 
vessels which catch rain water as it falls and hold it indefinitely. 
The leaf pitcher is smooth inside and is provided with a lip which 
has hairs over its surface pointing downward toward the interior of 
the vessel. The pitcher, partly filled with rain water, forms a 
trap for beetles and other insects that happen to get into it. Be- 
cause of the peculiar structure of the vessel, the hapless insect is 

FIG. 139. The leaves of Venus' fly trap are adapted to the capture and 

digestion of insects. 

unable to crawl up the sides, and as a result, it finally dies. In the 
case of the pitcher plant shown in Fig. 140, substances resulting 
from the decomposition of the insects are absorbed by the cells of 
the pitcher as organic food for the plant. Like other green 
plants, the pitcher plant is able to make food for its own use, 
but it also seems able to supplement this supply with that which it 
secures through the carnivorous habit. 

The sundews grow in situations similar to those in which pitcher 
plants are found. Each leaf has a definite petiole which holds a 
circular leaf blade. Numerous hair-like projections extend ver- 
tically upward from the upper surface of the blade, the ones at the 



edge being longer than those at the center. At the end of each 
projection there is a glistening droplet of a viscid substance which 
resembles dew in appearance. Insects, attracted by the sparkling 
droplets and wine-red color of the leaves, alight and stick fast to 
the hairs. This contact stimulates the leaf, which reacts by rolling 
the edges inward, thus bringing other hairs in contact with the 
insect and entangling it still more. The trapped insect dies, its 
body is digested by enzymes secreted by the hairs, and the organic 
food thus made available serves to supplement the supply which the 
sundew is able to make. 

Partial parasites. Some plants which are dependent upon 
other plants contain chlorophyll and can make a part or all of their 
foods. Mistletoe is a plant with chlorophyll which is a widely 
.^..^^^.^^^ distributed parasite on trees. 

The sticky seeds adhere to the 
branches of trees where they 
germinate. The young plants 
send their complex absorptive 
structures down into the tissues 
of the branch until they reach 
the conductive structures. Here 
they derive from the host raw 
materials, including water and 
mineral salts, and also a part 
of their food supply. Since 
-these plants are somewhat de- 
pendent but do not obtain all their foods from the host, they 
have been called partial parasites. 

Ability to withstand drying. You have probably noticed, as 
you walked in the woods, a slight coating on the north side of many 
trees, grayish green in dry weather and bright green during rainy 
periods. This is an alga known as Protococcus, the Indian 
compass plant, so called because the presence of masses of this 
plant on trees could always be relied upon by Indians in the woods 
as a guide to direction. In dry weather the cells are inactive. 
When rain comes they rapidly absorb water, begin to grow and 
multiply, and because of the large number of active cells they 
appear as a light-green coating on the bark. Many of the lichens 

FIG. 140. The pitcher plant, a car- 
nivorous plant. 


and mosses show this same ability to retain life during periods of 
extreme drying, to revive quickly when wet, and resume their 
normal life activities. The resurrection fern (Poly podium poly- 
podioides) may grow from the crevices of rocks or as an epiphyte on 
trees. It has the usual fern characteristics when moisture is suffi- 
cient, but as the drought approaches, the leaves wither and curl 
and the plant appears to be without life. When water again be- 
comes available, the leaves uncurl and immediately take on a 
bright green, and the plant begins a new period of activity. The 
Mexican resurrection plant (Selaginella) , sold in stores as a 
novelty, may remain dormant for months in a dried condition and 
still retain life. (See Fig. 147.) 

Exercise 120. Collect pieces of bark to which dried lichens are attached 
and pieces of bark with a coating of Protococcus. Moisten, place in a shallow 
dish with a little water, and cover with a bell jar or inverted battery jar. 
What change do you note in the appearance of the plants? Place in the light 
and observe for several days. What do you conclude as to the ability of the 
plants to revive after drought? 

Water loss in plants. Under ordinary conditions, a green plant 
is continuously losing water. In fact, water loss is one of the most 
serious of the problems connected with the growth of plants. 
Water plants which are submersed are not subject to danger from 
loss of water. In most swamp plants which have their roots in a 
soil saturated with water, excessive loss by evaporation is readily 
made good by increased absorption. In bog plants, however, and 
the plants of salt marshes, which have water in abundance, absorp- 
tion is difficult, and hence excessive water loss is destructive. The 
question is one of water balance. The amount of water lost by the 
plant through evaporation together with that used in food manufac- 
ture must not exceed the amount of water taken into the plant by 
absorption. In habitats where the water supply is deficient or 
where absorption is difficult, specialized structures are necessary to 
prevent excessive water loss. 

Why do plants lose water? Since water loss incurs danger to 
the plant, why should there be water loss? Suppose the plant were 
so well protected that there would be little or no loss of water to 
the outside. The deciduous tree in winter condition is almost 
completely impervious to water. At this time, however, the tree is 


comparatively inactive. The different plant processes are practi- 
cally at a standstill. The plant could not continue to live in that 
condition indefinitely. If you will recall the features of leaf 
structure which make food synthesis possible, you will remember 
that exchange of gases with the outside is made possible by open- 
ings, usually in the lower epidermis, the stomata. See Fig. 28. 
These holes in the epidermis of the leaf are an absolute necessity 
since carbon dioxide must enter the leaf as raw material for food 
synthesis, and oxygen must escape as a by-product of the same 
process. Besides, some oxygen must be available inside the leaf 
at all times to be used by the plant in the process of respiration. 
When the leaf is active the stomata are open and the water which 
evaporates from the moist cell surfaces into the intercellular spaces 
in the spongy tissue constantly diffuses into the atmosphere sur- 
rounding the leaf as long as the humidity of the air outside the leaf 
is less than that of the air inside. If the air surrounding the leaf is 
dry, evaporation and diffusion from the active leaf take place 
rapidly and water loss is great. If the air outside the leaf is 
saturated with water vapor, as it frequently is in the tropics, there 
is little water loss from the leaf. Thus, while water loss may be a 
menace to the plant under dry conditions, it can not be prevented 
if the plant is to function. The success of plants in many situa- 
tions depends upon their ability to develop protective structures 
which guard against excessive loss of water. The loss of water 
from plants by evaporation is called transpiration. 

How is the water supply of plants conserved? There is nothing 
in the environment of plants more variable than the water supply. 
It is an exceptional season if we do not have to sprinkle our lawns 
many times to keep the grass from drying up. At times, some 
means of conserving water is necessary to save the life of the plant. 
It may be necessary, even, for a plant to drop its leaves in mid- 
summer during a severe drought. In general, plants or parts of 
plants which are exposed to dangers of excessive drying develop 
protective structures which tend to prevent conditions that might 
be fatal to the plant. Sun plants are apt to be protected more than 
shade plants, and even in the same tree, the exposed upper leaves 
are more protected than the shaded lower ones. In the older leaf 
the outer walls are usually thickened by a deposit of a fatty sub- 


stance known as cutin. This aids in protecting the leaf against 
excessive water loss. Cutinization of the epidermis of the apple 
and other fruits tends to hold the water inside. Broad-leaved 
evergreens, as holly and magnolia, show especially heavy cutin de- 
posits which protect the plants during the colder seasons when ab- 
sorption of water from the soil is more difficult and excessive loss 
from leaves might be fatal. The leaves of the sedums (live- 
forever) and those of many other plants are coated with a layer of 
wax, known as " bloom/ 7 which prevents the escape of water. In 
spraying cabbage plants for protection against insect pests, soap 
must be added to the spray solution to dissolve the wax on the 
leaves. Otherwise, the spray solution rolls off the waxy leaf in 
large drops without wetting it and the insects are able to eat the 
leaf without getting any of the poison. 

Leaves of mullein, Shepherdia, and other plants are covered with 
hairs which prevent free movement of air currents and thus reduce 
evaporation. This condition seems also to reduce the absorption 
of heat by the leaf and in this way indirectly prevents water loss. 
There is a decided tendency toward leaf reduction in plants 
exposed to dangers of excessive transpiration. In the cactus, food 
synthesis occurs exclusively in the stem; the only leaves of the 
plant are small structures which appear on very young stems and 
are soon lost. In some of the euphorbias, natives of Africa, which 
are occasionally seen in our conservatories, there are true func- 
tioning leaves which are dropped at the approach of drought, a new 
crop appearing when rains bring about conditions which are more 
favorable. Frequently, in conditions of severe drought, our decid- 
uous trees lose many of their leaves, even in mid-summer. This is 
a protection against more severe injury which might result if trans- 
piration were not checked. 

Many plants which live in regions subject to drought conditions, 
as the aloes and cacti of the arid regions of the southwestern part 
of the United States, are fitted to these conditions by having thick, 
succulent leaves or stems. A large amount of water is stored in 
these structures when water is available. The plant is able to 
draw upon this store for use in maintaining the normal plant proc- 
esses when there is a scarcity of water in the soil. Purslane, a 
common weed of field and garden, has a succulent stem and succu- 


lent leaves which may hold water sufficient to keep the plant alive 
for days, even when uprooted. You may dig a purslane plant and 
cut it into pieces, and if these pieces lie on moist soil the various 
fragments will send out adventitious roots and produce a large 
number of separate purslane plants. This characteristic of water- 
retention is taken advantage of by florists in propagating certain 
varieties of begonia. Triangular pieces of the leaf, each containing 
a portion of a prominent vein, are set in sand. In this position 
they remain succulent and fresh until they have had time to de- 
velop roots and a bud, drawing upon the supply of food and water 
stored in the succulent leaf portion. After the adventitious roots 
and young shoot are well developed, the plants are transferred 
to pots of good soil where they soon become established and de- 
velop into sturdy begonia plants. 

In some plants, as in Aloe, the water-storage tissue is below the 
green tissues; in others, as in begonia, the water-storage tissue con- 
sists of the lower cells of a thickened epidermis. Stonecrop, which 
belongs to the former class, thrives on the scant water supply 
afforded by the thin layers of soil in the depressions of rock out- 
crops. The different sedums (live-forever) are used extensively 
in rock gardens and at other places for carpeting very dry, sandy, 
or rocky places in the open sun. 

Water absorption and water retention are especially difficult in 
plants living in salt marshes and on alkali soils where the salts in 
the soil solution are highly concentrated. Plants in these situa- 
tions must have a sap with a total concentration of solutes (solids 
dissolved) greater than that of the soil solution. In other words, 
the concentration of water particles in the soil must be greater 
than the concentration of water particles in the plant sap, other- 
wise water can not enter the root hairs of the plant by diffusion. 
This same principle seems, also, to be of importance in explaining 
the resistance of many plants to drought conditions. Protective 
structures of leaf and stem are important in preventing excessive 
water loss by transpiration, but the condition of concentration of 
cell sap which makes possible the absorption of water from com- 
paratively dry soil is probably of no less importance. 

What type of roots are plants likely to have if they are growing 
in a region where there are occasional light rains? What type of 


stem are these plants likely to have? What types of roots and 
stem is a plant likely to have in a region where there is regularly 
a period of fairly heavy rainfall alternating with a long rainless 

Problem 3. Why are certain types of plants found living together? 

What are plant communities? It is a source of interest to be 
able to study the vegetation from what we may call a bird's-eye 
view of the landscape. From such a study one of the first things 
that we discover is that vegetation is grouped according to habi- 

FIG. 141. Lily pond in a backyard. What different types of plant surround- 
ings are represented in this rock garden and lily pond? 

tats. There are ponds, swamps, flood-plains, sand hills, uplands, 
deserts, each habitat having a particular type of vegetation. A 
closer study reveals the fact that not only are certain species of 
plants found regularly in a certain type of habitat, but, in general, 
all the species found in a given type of habitat have many charac- 
teristics in common. Botanists have found that the most con- 
venient way to classify plants on the basis of habitat is according 
to their water relations. Plants found regularly in pond or swamp 
habitats are called hydrophytes; those found regularly in dry 
situations are known as xerophytes. By far the greatest number 



of different species are found growing regularly under conditions 
which may be considered intermediate between those of hydro- 
phytes and xerophytes. Plants which grow best under conditions 
of moderate water supply are called mesophytes. All the plants 

FIG. 142. Mesophytic woods of beech and maple trees carpeted by spring 

flowers. The soil is rich in humus. It is not wet, but the supply of moisture 

is usually sufficient during the entire growing season. 

of a habitat make up a plant community, and, classed on basis of 
water supply, communities are hydrophytic, xerophytic, or meso- 



Where oaks are found growing, one is likely to find hickories 
along with them. Then there are apt to be shrubs of witch hazel 
and spice bush, and such herbs as meadow rue and false Solomon's 
seal. These plants and many others are found growing together 
regularly in definite plant communities, and in habitats that are 
similar. Different grasses, rosin weeds, and blazing star are plants 
of the prairie community; Sphagnum moss, pitcher plant, sundew, 
and cranberry are plants of the bog community; and beech, maple, 
tulip tree, blood root, and dog-tooth violet are found regularly 

FIG. 143. The prickly pear, a xerophyte. The thick flattened structures are 
green stems with much water storage tissue. 

together in the mesophytic forest. The pioneer settlers of parts 
of our country were able to select lands which promised to be suit- 
able for certain crops which they desired to grow, by noting the 
type of plants which the land supported in the uncultivated state. 
Soil in the eastern part of our country on which was found a good 
growth of maple, beech, black walnut, and tulip tree was consid- 
ered ideal, when cleared of trees, for growing crops of corn, wheat, 
oats, and clover. 



Hydrophytic plant communities. Plants growing in ponds and 
lakes are subject to fewer and less abrupt changes than plants in 

any other habitat. Temperature 
is more nearly uniform in water 
than in air, and the water require- 
ments of the plant are satisfied 
at all times without any necessity 
for special provisions for absorp- 
tion or the prevention of water 
loss. The plants may be partly 
floating on the surface, or sub- 

. . _ , _ _ merged and rooted, or floating free. 

FIG. 144. Explain why there are no . . , , , , , , ,, 

Btomata on the under side of a Amon S the P lants that are USUa11 ^ 
water-lily leaf. Account for the waxy submersed are the pond weeds 
condition of the upper epidermis. (Potamogeiori) and water weed 

(Elodea) . Some of these frequently 

become a nuisance in park ponds because of their rapid and profuse 
growth. Many of the pond plants, as yellow pond lily, reproduce 
vegetatively by rhizomes in the 
mud. The exposed surface of 
floating leaves, as those of water 
lily, is usually coated with wax 
which prevents wetting of the 
surface and filling of the stomata 
with water. Submersed leaves 
are finely cut or ribbon-form 
and for that reason are not 
easily injured by water currents. 
Stems of water plants have 
little mechanical tissue. The 
plants do not need to support 
themselves to any great degree 
since they are held up by the 
buoyant force of the water. Air 

FIG. 145. The "knees" of cypress. 
The cypress is an conebearing tree 
growing in the swamps of southeastern 
United States. The roots are in very 
wet soil, and consequently there is 
not an ample supply of oxygen. The 
"knees" are growths of spongy tissue 
sent up from the roots into the air, 
and through them oxygen passes 
down to the roots. 

passes readily through the vege- 
tative structures of the plant 

within a system of air chambers and open spaces in the tissues, 
giving the plant buoyancy and facilitating the exchange of gases 


in the food-making and other tissues of the plant. Plants of 
the pond community succeed in their particular habitat because 
of their special fitness to live submersed, wholly or partly, in water. 
Young growing wheat or corn plants, if covered by a pond of water 
after a heavy rain, even if for only a short time, are killed. The 
wheat and corn plants are not fitted by structure to the conditions 
of a water habitat. Year after year plants migrate to the water's 
edge and hundreds of seeds are blown into the water, but still the 
pond community is limited in species to the comparatively small 
number which are fitted to the conditions which the pond affords. 

Swamp plants are similar to pond plants in some respects, hav- 
ing a reduced root system and prominent air chambers; however, 
in general characteristics they are more like mesophytes, particu- 
larly in leaf thickness, distribution of stomata, in amount and 
character of green tissue, and in protective structures. 

The peat bog is a peculiar type of swamp in which there are 
deposits of varying depth of partly decayed plant matter upon 
which vegetation is growing. The usual type is the Sphagnum bog 
in which the substratum is mainly dead Sphagnum moss. In the 
early stages the substratum is always saturated with water. 
Although some plants migrate into the bog from the outside and 
grow fairly well, in the main the plants of this community are 
peculiar to the bog, consisting of such species as Sphagnum, sun- 
dew, cranberry, pitcher plant, dwarf birch, poison sumac; and tam- 
arack. The plants of the bog are characterized by small leathery 
leaves and sparse root systems. Tamarack trees in the bog have 
roots only at or near the surface of the soil, but these trees planted 
in high ground become deep-rooted and show no tendency to 
remain at the surface. The saturated peat soil is unusual in many 
respects. The Sphagnum deposits are sour, and this has a ten- 
dency to prevent the growth of soil bacteria. This condition, along 
with the absence of oxygen, tends to prevent decay and other 
necessary soil reactions. Under these circumstances, acids and 
other toxic substances form in the soil, and there is a scarcity of 
soil salts in the bog waters. These conditions inhibit root growth 
and restrict the process of absorption. 

It seems odd that plants growing in soil saturated with water 
should have leaf structures similar to those of plants of dry regions 



which serve in preventing excessive water loss. In the light of the 
foregoing discussion, the need of protective structures of plants 
growing in bogs is evident. It has been found that tamarack trees 
and other plants of the bog will grow even better in situations out- 
side the bog under suitable ordinary conditions. Why, then, are 
they characteristic bog plants? The answer is found in the fact 
that the bog is highly selective. These plants, being tolerant of 
bog conditions, grow here because there is less competition. So we 
have in the bog a plant community made up of species which are 

FIG. 146. Lichens and mosses are among the first plants which are active in 
the process of transforming rocks into soil. 

Xerophytic plant communities. The deserts and dry plains of 
the southwestern part of the United States offer examples of habi- 
tats where water is scarce and plant growth is difficult. Plants 
must possess special structures to be able to withstand the severe 
conditions of these habitats. The root system is generally exten- 
sive and deep, the sparse, shallow root system of the cactus being 
a notable exception. The plants contain much water-storage tis- 



sue, or a highly developed cuticle, of a wax covering; or a plant 
may possess more than one or even all of these features which tend 
to prevent an excess of water consumption and water loss over 
water intake. Some of these plants either have no leaves, food 
synthesis being accomplished by modified stem structures, or there 
may be small leaves, or leaves which are easily dropped during 
extended drought periods. 

FIG. 147. Plants that can withstand drying. The lichens growing on this 

rock face and ferns clinging to the crevice are active during rainy periods. 

During times of drought they remain in a semi-dormant condition. 

Xerophytic plants have had an important part in the making 
of soil out of solid rock. Lichens and mosses are able to grow on 
rock surfaces when the rock is moist from rains, sending their 
absorbing structures (rhizoids) out into contact with the surface. 
These structures give off carbon dioxide, which with water forms 
carbonic acid. This slowly dissolves the rock. A part of the 
dissolved rock becomes raw materials for plants and a part goes 
back to rock in the form of very fine particles which, with the 
decaying plant bodies, form soil. As this process goes on, together 



with the action of water and frost, more and more soil is added 
until other plants which migrate into the community can get estab- 
lished, and finally the rock is covered with a layer of soil sufficient 
to support a rich mesophytic vegetation. As the habitat changed, 
migrants came into competition with the xerophytes and gradually 
crowded them out. So, as the plants change the habitat, they 
bring about conditions suitable for other plants which are better 
fitted to the new conditions, and the pioneers are eliminated. 

Here is an example of a plant suc- 
cession starting in a xerophytic 
habitat. Under mesophytic climatic 
conditions this type of succession 
goes through the various stages 
from xerophyte to mesophyte. 

Mesophytic plant communities. 
In a mesophytic climate such as the 
deciduous forest region of the 
United States, succession is always 
toward the condition in which the 
habitat is occupied by mesophytic 
plants. In the ordinary swamp and 
bog, development begins in a hydro- 
phytic habitat. As the succession 
continues through the swamp stage, 
there is a gradual development of 
a mesophytic community in which 
the plants require only a mod- 
erate water supply. On dry sand 
or rock surface where plants sur- 
vive only with difficulty, plant 
deposits, as leaves and other vege- 
tative parts, are added year after year, and these organic sub- 
stances, together with fine sand or clay, form a humus which 
has the property of retaining moisture. After long periods of 
time, the habitat, formerly extremely dry, has developed a soil 
which holds sufficient water to support a mesophytic vegetation 
consisting of such plants as beech, maple, jack-in-the-pulpit, blood 
root, and spring beauty. Thus, the types of plant communities 

FIG. 148. A tree Yucca in the 

Mohave Desert, California. A 

typical xerophytic tree. 



FIG. 149. Halophytes (salt plants) m 
the dune succession, Oceano, Calif. 

we see today in parts of our landscape which have not been altered 
by man are the results of changes which have been going on for 
many thousands of years. 
Plants fitted to the environ- 
ment have come, and as the 
environment changed these 
have gone and their places 
have been taken by others in 
a succession which has cul- 
minated in what we see now. 
Plant succession. Plants 
growing in a pond or lake 
tend to leave deposits which 
gradually fill up the depression 
and produce conditions which are unfavorable for the plants them- 
selves. There are usually swamp zones about the pond, and many 

of the plants in the zone next to the shore 
push out to the water's edge, and even 
into the water, by means of the promi- 
nent rhizomes which they possess. The 
swamp plants encroach upon the pond 
and by their deposits aid in eliminating it. 
Thus the pond with its typical commu- 
nity of plants gradually disappears and 
is followed in turn by the different stages 
of the swamp and finally by a mesophytic 
forest. As conditions change, plants of 
the habitat are displaced by migrants 
which are more suited to the new con- 
ditions, and so become established. 
Thus there is a series of more or less 
definite stages in the development of a 
region. The succession of plant com- 
munities which results from successive 
changes in plant habitats we call plant 

How has man affected plant succession? When man becomes 
a pioneer in a new region he cuts down the trees of the forest or he 

FIG. 150. Following lich- 
ens and mosses on rock ma- 
terials, first herbs appear, 
then trees and shrubs. Scene 
in Southern Illinois. 



breaks up the sod of the prairie or drains swamps to prepare the 
soil for growing his crops. He builds dams which cause streams to 
overflow their natural banks, causing the flooding of many acres 
of dry soil. These changes in the habitat of plants result in 
sudden changes in the different plant communities. 

A result of rapid water run-off sometimes brought on by 
forest destruction is the washing of much of the fertile humus 
from the hills to the lower grounds and into the streams. The 
fact that flood-plains and deltas are built up from the humus 

FIG. 151. Trees get a foothold in crevices of the rock and thrive in this 

difficult habitat. 

washed from higher lands accounts for the great fertility of the 
river lowlands and the increasing infertility of the hills. Thousands 
of farms in our country have been abandoned because the cultiva- 
tion of the soil on the lands is no longer profitable. 

Burned-over forests and abandoned farm lands represent 
habitats in which few plants but xerophytes can grow. Plants 
fitted to these severe conditions gradually come in and there is 
the beginning of a new succession. Herbs, shrubs, and trees 
come into the habitat, and these add humus to the soil and hold 



it in place. After many generations, if man allows the succession 
to go on, the final result will be a condition similar to that which 
man found when he arrived as a pioneer. Alabama, Mississippi, 
Michigan, Wisconsin, and other states have thousands of square 
miles of land which is being reforested in this natural manner. 

FIG. 152. A magnificent pine forest in Michigan. Forests similar to this one 
formerly covered extensive areas in the north central states. 

Man is the most destructive of the forces that affect plant suc- 
cession. Frequently through carelessness he starts a forest fire 
which sweeps over thousands of acres of territory, leaving destruc- 
tion of the work of centuries in its wake and changing a mesophytic 
plant habitat into one in which only xerophytes can dominate. 



Statistics for the year 1931 show that forest fires swept over 
52,000,000 acres in the United States in that year alone, with a 
money loss of $65,968,350. It has been estimated that approx- 
imately 50 per cent of forest fires are caused by locomotives, 8 
per cent by smokers, 0.1 per cent by camp-fires, and 11 per cent 
by boys. By proper education and care, man could prevent 
forest fires, or greatly reduce their number, except those caused by 
lightning. What factors of the native environment is it sometimes 

FIG. 153. A good stand of hardwood trees at the University of Illinois. 
Practical forestry is becoming more and more of a necessity. 

possible for man to change in a way to increase productivity when 
he begins to cultivate the soil? 

Exercise 121. Field study. How are the plants of a pond fitted to the 
conditions of the water habitat? If possible, study the conditions and plant 
life of a pond or lake; if this is not possible, answer as many of the questions 
as you can by a study of a well-stocked aquarium or lily pond. Note the 
characteristics of the plants which are submersed, paying attention to charac- 
ter of leaves as to texture, thickness, size, form, and shade of green; to the 
character of the stem as to size, amount of mechanical tissue, and whether 
covered by a protective coat of cutin, wax, or cork ; and to the roots, if 
present, noting whether they are water roots or soil roots. If both kinds of 


roots are present account for the presence or absence of root hairs on either 
water or soil roots. How do submersed plants' secure carbon dioxide and 
oxygen? Explain whether cutinization of leaves would be a benefit or hin- 
drance to submersed plants. Explain presence or absence of much mechanical 
tissue in submersed plants. Try to determine why the plants do not sink to 
the bottom. Describe features of any plants which you find floating on the 
surface of the water, as Riccia (a liverwort), duckweed, or water hyacinth. 
What advantage has a floating plant over a submersed plant? What disad- 
vantages to the plant are there in the floating habit? Study rooted plants 
with floating leaves, as water lily, noting wax coating of the exposed surface. 

FIG. 154. Forest fire destruction. Man's carelessness caused the destruction 
of a fine growth of young timber. 

Splash some water over these leaves and try to determine the r61e of the wax. 
Describe the light exposure of the water lily. Take water plants in closed 
cans back to the laboratory with you for further study. Write a full account 
of your field trip, answering the question at the head of the exercise. 

Exercise 122. Field study. How are swamp plants fitted to the conditions 
of the habitat? Visit a swamp at the edge of a pond, if possible. Study emersed 
plants, as bulrushes, arrow head, and water plantain. Examine the interior 
of portions of the plants, as stem and leaf petiole. What characteristics have 
these swamp plants in common with emersed pond plants? Explain. Study 
the plants of the zones of the swamp seen as one goes out from the pond. These 
represent stages in the development of the habitat from the hydrophytic to 



the mesophytic condition. Note the large number of plants with vertical 
leaves, as cat tails, reeds, and sedges. What is the advantage of this type of 
leaf in places where vegetation is dense? Remembering that the soil of the 
swamp in the earlier stages is saturated with water, how do you account for the 
fact that the roots of the plants extend in a horizontal direction near the 
surface, and some even extend upward? In so far as you are able to make this 
study of the swamp, try to answer the question at the head of this exercise in a 
clear and concise statement of the results of your investigations. 

Exercise 123. Field study. How are plants fitted to the conditions of a 
xerophytic habitat? Study the plants of any dry habitat, as a dry prairie, sandy 
hill slope, or railroad embankment. Note the character of the leaves of the plants, 
as to thickness, texture, color, and size. Break some of the stems. Are they 

FIG. 155. Why are our wild flowers disappearing? Here is one answer. 

succulent and brittle, or are they hard and tough? Determine the character 
of the roots by digging up some of the plants. Describe any tendency to the 
development of thorns and spines, or hairy leaves, or the rosette habit? Do 
you find any compass plants, as wild lettuce or rosinweeds? Write a detailed 
account of your study, giving your opinion as to why the plants which you 
found were able to become established in this xerophytic habitat. 

Exercise 124. Laboratory study of a mesophyte. In our study of plants in 
previous units, we have considered the mesophyte as our typical plant. It 
remains for us here only to consider the characteristics of mesophytes which 
distinguish them from hydrophytes and xerophytes. Select any available 
mesophytic plant of the greenhouse or garden, as geranium, bean, or four 
o'clock. Note the character of the leaves as to color, size, thickness, texture, 
and cutinization. Study Fig. 138, showing sections of the leaves of a hydro- 


phyte, of a xerophyte, and of a mesophyte. Compare the structure of the 
three types of leaves, paying special attention to cuticle, upper and lower 
epidermis, palisade tissue, spongy tissue, and occurrence of stomata. How do 
mechanical tissues of the stem of mesophytes compare with those of hydro- 
phytes and with those of xerophytes? Write a summary comparing the fea- 
tures of mesophytes with those of hydrophytes and with those of xerophytes. 

Problem 4. How are plants related by structure to the process 

of pollination? 

Pollination has been defined as the transfer of pollen from the 
anther of a flower to a stigma. The stigma receiving the pollen 
may be in the same flower with the anther producing the pollen 
or it may be in another flower. When pollen is transferred from 
the anther to the stigma of the same flower, the process is called 
close pollination. When close pollination is effected by contact 
of stigma and anther, it is called self-pollination. When the trans- 
fer of pollen is from the anther of a flower to the stigma of a flower 
on another plant, the process is termed cross-pollination. A con- 
dition intermediate between close-pollination and cross-pollination, 
in which pollen is carried from a flower to another flower on the 
same plant, is sometimes classed with cross-pollination but, in 
reality, it is more nearly related to close pollination. 

Most flowers seem to be fitted to the process of cross-polli- 
nation. It has been noted that some flowers are especially 
suited to cross-pollination by certain insects, as red clover by 
bumblebees. Many other examples of pollination by certain 
animals may be cited. In connection with this, the student should 
review the interrelations of plants and animals discussed on 
pages 238-242. The long bill of the humming-bird easily reaches 
the nectar at the bottom of the long spurs of the columbine, which 
is out of reach of the mouth-parts of bees, so the humming-bird is 
the principal agent in cross-pollination of the columbine. The 
long proboscis of the hawk moth, hovering before the Nicotiana 
flower, is uncoiled and thrust down into the tube of the corolla, 
at the base of which there is an abundance of nectar which is used 
by the moths as food. The relationship between Nicotiana and 
the hawk moth is seen further in the fact that the moths are active 
only in the evening or at night and they are aided in finding the 



flowers by the light color of the corolla and the marked fragrance 
of the blossoms. On the body and legs of insects are hairs and 
bristles to which pollen may stick and be carried from flower to 
flower. Indeed, the relation between the structures of flowers 
and the structures of insects is so noticeable that biologists believe 
that flowers and certain insects developed together in their changes 
in form. 

Why do insects visit flowers? It is a well-known fact that 
many insects secure nectar from flowers, using it for food on the 
spot, as butterflies moths, and certain flies, while others, as bees, 

FIG. 156. Columbine flower. 
Only humming birds or but- 
terflies and moths can reach 
the nectar in the tip of the 

FIG. 157. The hind legs of the honey 
bee are well constructed for the collec- 
tion of pollen. (From California Agri- 
cultural Experiment Station Bui. 517.) 

lay by a store for future use. Bees collect nectar, not honey, 
from flowers. This substance, which contains only from 15 
to 40 per cent of sugar, is lapped up by the mouth-parts 
of the bee and transferred to a honey sac near the stomach, in 
which it is held until the bee reaches the hive. Here the nectar is 
placed in cells and left until evaporation of water changes it into 
honey with a very high percentage of sugar. Bees also gather 
pollen and store it temporarily in cells, to be used later as food, 
chiefly by the larvae or young bees. Bumblebees and honeybees 



are the most efficient pollinators among the insects. This is due 
partly to their unceasing activity, and partly to their habit of con- 
fining their visits on the same trip and on many succeeding trips to 
flowers of the same species. " White clover " honey as offered by 
apiarists is produced in the white clover season and may really 

FIG. 158. Nectar glands sometimes occur on 
the leaves of certain acacias. These glands ap- 
pear as small protuberances. (From California 
Agricultural Experiment Station Circular 62.) 

FIG. 159. Pelican 
flower (Aristo- 
lochia), a carrion- 
scented flower, at- 
tracts flies as pol- 
linating agents. 

have been made almost exclusively from nectar secured from white 
clover blossoms. 

Nectar is secreted by special structures of the flower known as 
nectaries which are exposed in some flowers and hidden in others. 
In flowers having exposed nectaries the nectar is usually accessible 
to flies and other insects without specialized mouth-parts. In 



specialized flowers having concealed nectaries, the nectar can be 
obtained only by insects with mouth-parts modified to form some 
type of proboscis. 

In some flowers, as those of the poppy and nightshade families, 
there is little or no nectar. Insects visit these flowers only for 
pollen, which is usually produced in great abundance. In some 
cases pollinating insects visit flowers for sap, and in others, for 
protection. Certain flowers, as pelican flower (Aristolochia 
grandiflora) of conservatories, and our native carrion flower 
(Smilax herbacea) attract different species of scavenger flies by an 

odor resembling that of decaying 
flesh. These flies may get nothing 
from the flowers, but serve as efficient 
pollinators as they are apt to visit 
only flowers which are carrion-scented, 
and these are likely to be of the same 
species. (See Fig. 159.) 

How is cross-pollination brought 
about? There are two main types of 
flowers with regard to pollination, 
wind-pollinated flowers and animal- 
pollinated flowers. Wind-pollinated 
flowers usually have neither showy 
structures nor marked fragrance. 
Light, dry pollen is produced in great 
abundance, and it may be carried many 
The receptive structure of the female 
flower, the stigma, is feathery in form and, when ready for pollina- 
tion > is covered by a sticky secretion which catches and holds any , 
pollen grains that happen to reach it. In a species having pollen 
and ovules borne on different plants, as in the cottonwood, only 
cross-pollination is possible. In other cases the pollen which 
reaches the stigma may be from a flower on the same plant. 

It is significant that the showy and fragrant flowers are animal- 
pollinated. Differences in flower color and flower structure seem to 
have a direct relation to the process of pollination, usually to the 
process of cross-pollination. In some flowers, Iris, for example, and 
the orchid known as lady's slipper, the flower parts are so arranged 

FIG. 160. Orchid flowers are 
peculiarly fitted to cross- 
pollination by insects. 

miles by a strong breeze. 



that the insect bearing pollen on its body enters the flower in such a 
manner as to rub against the stigma, and upon leaving, it rubs against 
the anther. Thus, pollen deposited on the stigma is from another 

A very common method of preventing close pollination is 
by successive maturing of the stigma and anther of the same 
flower; the pollen may be shed before the stigma is mature, or 
the stigma may mature before the pollen grains. Insects, in going 
from flower to flower, carry mature 
pollen from a flower with an immature 
stigma to another flower having a 
mature stigma. Here close pollination 
is impossible and cross-pollination is 
likely to occur. 

In some plants, as flax and prim- 
rose, there are different types of flowers. 
Some have long styles and short 
stamens, while other plants of the 
same species have short styles and 
long stamens. An insect visiting the 
former type of flower receives pollen 
on the front part of the body and 
leaves pollen from another flower pre- 
viously visited on the stigma from 
the back part of the body. When 
this insect visits the latter type of 
flower, the front part of the body, 
which is covered with pollen, comes in 

contact with the stigma and the back part of the body receives 
more pollen from the long stamens. 

Sometimes pollen will not germinate on the stigma of the 
same flower or even on the stigma of any other flower on the same 
plant; or pollen may not germinate as readily on a stigma of the 
plant which produced it as on the stigma of another plant. In 
the former case, close pollination is impossible; in the latter, 
cross-pollination is more likely to occur. 

One of the simplest features of flowers which tends to favor 
cross-pollination and prevent close pollination is the long style 

FIG. 161. The moccasin 
flower, an orchid, a flower 
peculiarly fitted to cross-pol- 
lination by insects. 



of many flowers which brings the stigma out above the stamens. 
The visiting insect, as it enters the flower, is apt to brush over 
the stigma, in its exposed position, and leave pollen which it has 
brought from another flower. Also, pollen from the anther of 
the same flower can not fall upon the stigma. 

Many examples of highly specialized relationships between 
flowers and insects might be given. The pollination of the Smyrna 
fig is the most remarkable of the known examples of cross-pollina- 
tion. The details of this process have been noted. (See page 240.) 

Pollination between anther 
and stigma on the same plant. 
Although we have been accus- 
tomed to thinking that there 
must be marked advantages in 
cross-pollination and judging 
from the large number of struc- 
tural features that fit flowers to 
this process, and from the results 
of investigation, we have good 
grounds for this belief yet pol- 
lination between anther and 
stigma on the same plant is 
quite common. Indeed, some 
flowers possessing features which 
favor this type of pollination 
are about as specialized as those 

FIG. 162. Insect pollination. This 
butterfly is procuring nectar from the 
milkweed blossoms, but it is also 
carrying pollen from flower to flower. 

which we have noted as favor- 
ing cross-pollination. In the 
composite family, each so-called 
flower is really a head of a large number of flowers, usually of two 
kinds, ray flowers around the edge and disk flowers in the center 
of the head. As a rule, in the sunflowers, only the disk flowers 
produce seeds. Some composites, as the dandelion, mature all 
the flowers of the head at the same time, but in the greater 
number, the flowers mature and are ready for pollination from 
the outer edge of the disk inward, and pollination of all the 
flowers of a head may require a week or longer. The anthers 
of a given flower mature before its stigma. As the stigma ma- 


tures, it elongates and comes in contact with anthers of other 
flowers of the head. Thus, pollination between different flowers 
on the same plant is accomplished, and judging from the vigor 
of the plants and from the very large number of seeds produced 
by such composites as wild lettuce, common thistle, dandelion, 
and sunflower, their method of pollination is very efficient. The 
pollination of Yucca by the Pronuba moth furnishes an example 
of highly specialized close pollination. (See page 240.) Certain 
flowers (cleistogamous flowers), as some of the flowers of violets, 
never open. In these flowers seeds are produced regularly in 
abundance as a result of close pollination. In flowers of this type 
in which close pollination results from contact of anther with 
stigma, we have examples of true self-pollination. (See Fig 82.) 

Exercise 125. What are the essential parts of a flower? Review your 
study of a flower made in Unit V. Identify in several flowers of different 
species the anther and filament of the stamen, and the stigma, style, and ovary 
of the pistil. What is the role of each of these parts in pollination and fertili- 
zation? Make sketches to show the relative size and arrangement of the pistil 
and stamen in one flower. Note the position in the flower of the anther in 
relation to that of the stigma. 

Exercise 126. Where in the flower is pollen produced? Study under a 
microscope (binocular, if available), without water or cover-slip, anthers 
from different flowers of the same species, one from a flower in the bud, 
another from a flower just opened, and a third from an older flower. How 
does pollen get out of the anther? Does the pollen seem sticky and suitable 
for being carried away by animals, or is it light and dry and suited to wind 
dispersal? Does there seem to be any certain time when the pollen is ripe 
and ready for pollination? Can pollen be carried away from a flower before 
it is ripe? Make careful notes, explaining your answers to the questions of 
the exercise, and make a sketch to show how pollen is set free from the pollen 
sacs of the anther. 

Exercise 127. How do flowers receive pollen? Study several stigmas 
under the microscope (binocular preferred) of flowers fully opened. Find 
pollen which has been transferred to the stigma. What makes it stick to 
the stigma? How is the stigma different from the style? Find stigmas which 
seem to be too young to receive pollen. Find others which are too old. What 
would happen if ripe pollen grains were to touch a stigma either too young or 
too old. What are two r61es of the sweet, sticky liquid on the surface of a ripe 
stigma? Explain whether close pollination can occur if the stigma of a flower 
gets ripe before its pollen, or if the pollen of a flower gets ripe before its stigma. 
In what way is it possible for close pollination to take place in these two types 
of flowers? 


Exercise 128. Types of flowers with reference to pollination. The 
flowers of this study will need to be chosen from suitable specimens, represent- 
ing the different types, which are available at the time the study is made. 

Open type flower, as buttercup or nasturtium. Note the relation of anther 

FIG. 163. Maize, or Indian corn. At left, pistillate inflorescence, or "ear"; 
at right, staminate inflorescence, or "tassel." (From Robbins, in Botany of 

Crop Plants.) 

and stigma in position. Is the plant monoclinous (stamens and pistil in the 
same flower) or diclinous (stamens and pistils in separate flowers)? Describe 
the flower with reference to odor, color, whether nectar is present, and whether 
it is regular (floral leaves similar) or irregular (floral leaves unlike). Describe 


the features of the flower which fit it for pollination by wind, insects, contact, 
gravity, or water. Try to determine whether close pollination or cross- 
pollination is more likely to occur. Make a drawing of the flower to bring out 
pollination features. 

Specialized flower. Choose for this study a sympetalous (petals united) 
flower, as butter-and-eggs or snapdragon. Make a drawing of the flower with 
the floral parts in their natural position. Cut away enough of the flower to 
show the position of the pollination structures. Is the flower fitted to polli- 
nation by wind or by insects? Give reasons for your answer. What struc- 
tures are present which would attract insects? If insect-pollinated, what 
insects would be suitable as pollinators? Would cross-pollination or close 
pollination be more likely to occur? Write a summary of the features of the 
flower which show specialization, and explain how this specialization is an 
advantage to the plant. 

Composite flowers. Use any available composite, as yarrow, Coreopsis 
or sunflower. Note that the so-called flower is in reality not a single flower, 
but a head of a large number of flowers set upon a flat receptacle and sur- 
rounded by green leaf -like bracts. Separate the head vertically by breaking it 
open through the middle of the disk. Make a drawing of the exposed flowers 
to show their relation to each other in natural position. The flowers at the 
outer margin of the disk mature first, then the other flowers mature gradually 
from outside to center. The anther matures and sheds its ripe pollen before 
the stigma of the same flower is mature and ready to receive pollen. The 
pollen is pulled out of the corolla tube by the hairy style as it pushes out, 
bearing the immature stigma. The flowers being so near together, mature 
stigmas of other flowers come in contact with this ripe pollen, bringing about 
pollination. Insects visiting the head of flowers aid in the transfer of pollen. 
Pick out suitable flowers of the head, and try to make out the different pollina- 
tion features outlined above. Sketch under a dissecting lens or binocular 
microscope disk flowers in different stages of development, one before the 
stamens appear, another at the time the pollen is ripe, and a third at the time 
the stigma is mature. 

Problem 5. How are fruits and seeds fitted to the process of 
dispersal of plants? 

According to popular usage, the term fruit clearly includes 
such plant structures as peaches, apples, blackberries, and pine- 
apples; tomatoes, peppers, and cantaloupes, on the other hand, 
are ordinarily classified as " vegetables. " To a botanist, the 
term fruit has a much wider meaning. In botany, a fruit is a 
ripened ovary, together with any other structures that may have 
developed with it. Pollination and fertilization usually result ia 


the development of the seed, together with the development of 
the other structures of the fruit. A few fruits, as the banana and 
certain oranges and grapes, develop without seeds. Fruits may 
be either fleshy, as grapefruit, or dry, as a grain of corn. In both 
kinds, however, the principal roles of the structures which enclose 
the seed are protection and dispersal. 

How are the fleshy fruits fitted to dispersal? The apple, pear, 
crabapple, and quince belong to the group known as pome fruits. 
The ovary is the core, the ovary walls being the hard plates sur- 
rounding the seeds. The fleshy part of the apple is the specialized 
receptacle which has grown up and around the ovary. During 

FIG. 164. The fruit (drupe) of peach with the single seed surrounded by the 
ovary wall in two layers, the inner one forming the " stone " and the outer 

layer the pulp. 

the time of development, the fleshy part contains starch and is 
quite sour, because of the presence of malic acid and the absence 
of sugar at this time. The green fruits are hard, the cells being 
held together by a substance which is mainly calcium pectate. 
As the seeds become mature, starch is changed to sugar and the 
quantity of malic acid may be reduced somewhat. At the same 
time, the pectic compounds are being broken down into other 
substances with the result that the cells are separated to some 
extent and the fruit becomes mealy. The changes in the fruit 
which make it edible as the seeds develop suggest the role of seed 
dispersal by animals that use the fruit as food. Man has been 


especially active in propagation and distribution of these fruits 
because he has found them desirable as valuable food supplies. 

One of the most common of the fleshy fruits is the drupe, or 
stone fruit, which includes the peach, plum, cherry, apricot, and 
olive. The seed is enclosed in a stony layer which we ordinarily 
consider a part of the seed structure, but which, in reality, is not 
a part of it. Around this stony layer is a second layer which is 
fleshy. In all these fruits the stony covering of the seed serves 
as a protection. The smaller fruits, as cherries, may be swal- 
lowed, stone and all, by the larger birds and pass through the 
alimentary tract without injury to the seed. The chances are 
favorable that many of the seeds will be dropped in a suitable 

FIG. 165. The tomato, a berry. The seeds are enclosed in the fleshy ovary 


place for germination and growth at some distance from the 
parent tree. 

The berry has a fleshy wall enclosing seeds. Berries include the 
fruits of such common plants as the tomato, currant, grape, blue- 
berry, and cranberry. Seeds of these fruits are small, and many 
of them are distributed uninjured by the animals that use them 
as food. The pepo is a berry with a hard rind, for example, 
squash and cucumber. The hesperidium is a berry with a leathery 
rind, as the lemon and orange. 

The blackberry is not a real berry in the botanical sense, but 
is a body formed by a large number of separate ovaries, each a 



tiny drupe, attached to a single receptacle. The raspberry is 
similar to the blackberry, but the mass of fruits becomes detached 
from the receptacle when ripe. The strawberry is really not a 
fruit at all, but a fleshy receptacle bearing numerous tiny achenes, 
containing the seeds, on its surface. Each of these achenes is, 
in reality, a tiny fruit. Fruits of this type are known as aggregate 

The mulberry and pineapple are formed from many individual 

flowers all fastened tightly 
together, and for this reason 
are called multiple fruits. 
The Smyrna fig is a syco- 
nium, consisting of a fleshy, 
hollow receptacle, the one- 
celled ovaries developing into 
nutlets which are embedded 
in the inside wall. 

The fruits mentioned in 
the foregoing discussion either 
have fleshy ovary walls or 
are developed in connection 
with other fleshy flower parts 
which are edible. Most of 
them are characterized, in 
addition, by being attrac- 
tively colored, shades of blue, 
red, and yellow being espe- 
cially prominent. Practically 
all of them are green in color and inconspicuous before maturity, 
becoming edible and showy as the seeds ripen. Because of the 
possession of these features, animals have a prominent role in 
the dispersal of many of the plants which produce these attractive, 
fleshy fruits. 

How are dry fruits fitted to dispersal? The dry fruits are of 
two types, dehiscent (splitting open when ripe) and indehiscent 
(not splitting open). 

Dehiscent fruits. Among the dry, dehiscent fruits, the legume 
is a common example. The pod of the garden pea or bean shows 

FIG. 166. The aggregate fruit of the 

raspberry, made up of many separate 

fruits massed on a single receptacle and 

developed from a single flower. 



the characteristics of this type of fruit. You can not help noting, 
in shelling peas, the resemblance of the opened pod to a leaf, the 
outcurved edge of the pod being the midrib. You have probably 
noted that the seeds are fastened to the pod at the incurved suture. 
The opened pod shows that the legume is, in reality, a modified 
leaf, in this case a single carpel bearing seeds at the edges. When 
the legume dries, it usually splits open at the two sutures, the two 
halves of the carpel curling in 
such a way as to expel the seeds. 

The follicle is a dry fruit 
developed from a single carpel 
which opens along one suture. 
You may have noted in your 
garden the opening of the 
follicles of larkspur, or those 
of columbine, at the top and 
along the inner edge in such a 
manner that the dry seeds are 
thrown some distance as the 
tall stem is swayed about by 
the wind. The fruit of the 
poppy or that of the violet is 
an example of a capsule. Cap- 
sules open in different ways, 
allowing the seeds to drop or 
be thrown out by movements 
caused by the wind. The silique 
is made up of two carpels which 
open at maturity, the two valves 
curling upward leaving a parti- 
tion from which the dry seeds 

become detached. Most of the members of the mustard family 
have this type of fruit. 

Dry indehiscent fruits. A very common example of a dry 
indehiscent fruit is the achene, represented by the fruit of the 
sunflower and by that of the buttercup. The single seed is 
attached to the ovary at one point only. A grain of corn, typical 
of the fruit of the grasses, is a caryopsis. Corn meal, as you buy it, 

FIG. 167. Multiple fruit of the pine 
apple. The fruit is developed fron* 
the ovaries of many separate flowers. 



consists of granular bits of the endosperm and embryo of the 
seed, the tough fragments of ovary wall and the testa having been 
sifted out in the process of milling. The samara or key fruit has 

the ovary expanded in such 


Examples of plants pro- 
ducing this type of fruit 

significance of the fact 
that winged fruits are 
common among trees? 
The nut, as the acorn, 
chestnut and hazelnut, is 
similar to an achene, but 
has a hard outer wall. 
Though nuts are eaten in 

large numbers by animals, many that are buried by squirrels 
are never found, and so are in a suitable place for germination. 

FIG. 168. The 

fruits (pods) of Lima 

FIG. 169. Fruits (pods) of the black locust. 



It is possible, also, that many fruits of oak, hickory, and walnut 
are carried away in time of flood and deposited by water on a 
bank where germination and growth of the seeds may take place. 

By what means are fruits and seeds dispersed? Dispersal by 
propulsion. It has been noted in a previous section that legumes 
disperse their seeds by a curling of the two halves of the dried pod 
in such a way as to throw the seeds some distance. The well- 
known garden balsam or touch-me-not has a pod which suddenly 
explodes upon being touched 
and a violent curling of the 
carpels throws the seeds out 
with considerable force. 
When one walks among jewel 
weeds (Impatiens) in the 
woods in late summer, one can 
hear seeds falling all about as 
a result of the bursting of the 

Dispersal by water. Many 
seeds have walls that are 
not readily penetrated by 
water. These may retain 
viability for a long period 
while being -carried great dis- 
tances by water. As the water 
recedes, many seeds are left 
in the silt of flood-plains and on 
banks where they have suitable 
conditions for germination. 

Dispersal by animals. The farmers of the country suffer huge 
losses every year on account of burs which get into the wool of 
sheep and reduce its value. Fruits with reflexed spines and barbs, 
as burs of cocklebur and burdock, beggar ticks, and Spanish 
needles, cling to the furry or woolly coats of animals and to the 
clothing of man and may be carried miles from the plant which 
produced them. Our worst weeds have been brought into the 
United States from abroad by man on his clothing, in his luggage, 
and in farm and garden seeds that he has imported, and they have 

FIG. 170. The fruits of burdock are 

provided with hooks which fasten the 

fruits to passing animals. 



FIG. 171. Dispersal by the wind. Fruit (samara) of white ash. 

been scattered throughout the country over man's highways and his 
railroads. We have already noted the dispersal of seeds by birds that 
eat fleshy fruits and drop the seeds in places suitable for growth. 

Dispersal by wind. If you 
live near cottonwood trees, you 
have noticed, in late spring, 
" cotton 7 ' flying about. If you 
have examined some of the cotton, 
you have noticed that tiny seeds 
are attached to the small tufts. 
In this way seeds are carried by 
the wind. Dispersal by the wind 
is wasteful, but it is effective. 
Seeds of some of our most trouble- 
some weeds, as wild lettuce and 
Canada thistle, are scattered far 
and wide by means of wind which 
FIG. 172. Fruit dispersal. Fruit lifts the parachute attached to 
heads of the composite, goat's tne see( j an( j carries parachute 
baud. The opened head at the d h h ^ ^ d 

left shows the fruits, each with 

its parachute, ready to be lifted possibly for miles before they are 
and carried away by the wind, dropped. Various states have at- 


tempted to cope with the Canada thistle menace by enacting laws 
requiring owners of land to keep all thistles mowed before they 
blossom in order to prevent any production of seeds. The 
tumbleweeds, as tumble mustard, winged pigweed, and Russian 
thistle, branch in such a way as to form a large spherical mass 
which, when mature, breaks off near the ground and goes tumbling 
before the wind, scattering ripe seeds as it goes. When we 
realize the possibilities of wind dispersal of plants, we are re- 
minded that^ weed prevention on our premises is not only profit- 
able for ourselves, but is also a civic duty, since weeds are no 
respecters of fences and, if allowed to grow in our own fields 
and gardens, are sure to spread to those of our neighbors. 

Exercise 129. What is a fruit? Examine fruits of the following list and 
make sketches, labeling the parts of the flower, as ovary, style, receptacle, 
stigma, represented in the developed fruit: bean, grain of corn, sunflower, 
prune (soaked for study), maple. 

Explain whether the definition of a fruit as " a ripened ovary " holds for 
the fruits mentioned above. 

Examine and sketch in the same way the following fruits: apple, pineapple, 

Explain whether the definition of a fruit suggested above holds for these 
fruits. If it does not, use your labeled sketches in revising the definition to 
include all fruits. 

Exercise 130. What are the different types of fruits? Using your 
sketches made in the previous exercise, together with your text, classify the 
fruits in the lists of Exercise 129 and others, and give the characteristics of each 
type of fruit, as: Bean, legume or true pod dry, dehiscent; one carpel, 
splitting along two sutures. 

Exercise 131. How are fruits and seeds dispersed? Study available 
fruits to determine probable means of dispersal and describe structural features 
which aid, using sketches where desirable. Make lists under the headings as 

1. Dispersal by propulsion, as sweet pea, witch hazel. 

2. Dispersal by attachment to animals, as cocklebur, burdock. 

3. Dispersal by means of indigestible seeds of fleshy fruits that are eaten 
by animals, as raspberry, black haw. 

4. Dispersal by wind, as dandelion, ash, tumbleweed. 

5. Fruits and seeds without obvious means of dispersal, as acorns. 

Suggested activities, (a) Make a collection of fruits found in the market 
and classify on basis of type; as apple a pome fruit. 

(6) Make collections of dry fruits and classify on basis of means of dis- 



1. Explain why the stem of a plant placed in a horizontal position in 
darkness will grow upward at the tip. 

2. What will happen if onion sets are planted upside down in the soil? 

3. Explain why gladiolus corms will send out roots and shoots more 
quickly if the dry scales and other dead parts are removed from the corms be- 
fore planting. 

4. Why do celery stalks (leaf petioles) grow taller if the soil is banked up 
around the plants? 

5. Why is celery more crisp and tender when banked with earth than when 
allowed to grow without banking? 

6. In selecting strawberry plants, why is it safer to get plants from a 
neighbor who has a successful strawberry patch than to send away to another 
part of the country for plants of an advertised, fancy variety? 

7. If a farmer is moving to a distant part of the country in about the same 
latitude, he might take farm seeds along with him, or he might wait and get 
seeds from farmers in the new location. What would be your advice? Ex- 

8. What is the advantage of the deciduous habit? 

9. Why is it necessary for the leaves of the evergreens of our colder 
regions to have xerophytic structures? 

10. Why is it a good plan to give the soil about the roots of ornamental 
evergreens a good soaking with water on the approach of cold weather? 

11. In growing plants in a region new to you, what use could you make of 
information concerning the native wild plants growing in the vicinity? 

12. Should a farmer who raises red clover permit the boys to destroy 
bumblebees' nests? Explain. 

13. Why do gardeners have a hive of bees in a greenhouse where cucumbers 
are grown? 

14. Explain reasons for the practice of spraying fruit trees with poison, 
for destroying codling moth, once just before the blossoms open, and a second 
time just after the petals have fallen, but avoiding spraying while the trees are 
in full bloom. 

15. What do honeybees gain from buckwheat blossoms, and what does 
buckwheat gain from visits of honeybees? 

16. Of what advantage is it to the carrion flower to possess an odor similar 
to that of decaying flesh? 

17. What kinds of flowers cannot be close pollinated? 

18. What kinds of plants cannot be cross-pollinated under natural conditions? 

19. What advantage is it to the species to possess flowers in which both 
cross- and close pollination are possible? 

20. In what way might wading-birds carry seeds and place them in a loca- 
tion suitable for germination? 

21. Why are some of our most persistent weed pests found among the com- 


22. Explain why saturating the soil about the roots of Canada thistle with 
strong salt solution will kill the plants. 

23. Explain why ragweeds appear in the stubble of grain fields, although 
few weeds were noticed while the grain was standing. 

24. Why is it necessary to dig weeds from a newly seeded lawn although 
little weeding is needed in an established lawn? 


House Plants, by PARKER T. BARNES, published by Doubleday, Doran and 
Company, New York, 1909. 236 pages, 31 illustrations. This gives direc- 
tions for the growing of house plants, including their selection, soil preparation, 
seed sowing, potting, propagation, and other operations. 

Plant Ecology, by JOHN E. WEAVER and FREDERIC E. CLEMENTS. Pub- 
lished by McGraw-Hill Book Company, New York, 1929. 520 pages, 262 

Familiar Flowers of Field and Garden, by F. SCHULER MATHEWS, pub- 
lished by D. Appleton-Century Company, New York, 1903. 308 pages, 
numerous illustrations. 

Insectivorous Plants, by CHARLES DARWIN, published by D. Appleton- 
Century Company, New York, 1899. 462 pages, 30 figures. This book is a 
classic on insectivorous plants. It describes in detail the characteristics and 
behavior of the sundew, the bladderwort, Pinguicula, and other insectivorous 

Plant Ecology, by W. B. McDouGALL, published by Lea and Febiger, 
Philadelphia, 1927. 326 pages, 114 figures. This discusses the environmental 
factors, plant communities, plant succession, phenology, symbiosis, pollination, 
and the ecology of roots, stems, and leaves. 

Manual of Weeds, by ADA GEORGIA, published by the Macmillan Com- 
pany, New York, 1914. 593 pages, 386 illustrations. A description of all the 
most pernicious and troublesome plants in the United States and Canada, 
their habits of growth and distribution, with methods of control. 

Weeds, by W. C. MUENSCHER, published by the Macmillan Company, 
New York, 1935. 577 pages, 123 illustrations. Discusses dissemination and 
importance of weed, weeds of special habitats, weed control, and describes 
the important weeds of the United States. 


One of the fundamental laws of nature is that life comes from 
life. Man has been able to do wonderful things in the chemical 
laboratory, but he has not been able to produce any substance 
with the properties of living material. Geology teaches that the 
earth has gone through a series of changes in its development, 
and that life has existed on the earth during at least half of its 
geologic history. 

Much is known of the early plant forms from studies of their 
fossils. These records also reveal something of the story of the 
development and disappearance from the earth of great plant 
groups as well as the rise and development of those which have 
become the dominant plant groups of today. 

Primitive man was able to use plants in many different ways 
in his daily life. They were food, shelter, medicine to him, and 
they beautified his landscapes. The same laws which were in 
operation in nature changing plants in the wild were also changing 
the plants which man had brought under some degree of cultiva- 
tion and control. Man early learned to take advantage of the 
changes in plants which made them more suitable for his uses, 
and he became a plant breeder. It is true the methods he used 
were haphazard at first, but in the end they were effective in 
securing better plants to meet his needs. 

It had long been known that plants tend to be similar to their 
parents, that is, that certain characteristics are inherited by 
offspring. It is also known that no two plants are exactly alike, 
that offspring tend to be different in certain respects from their 
parents. In other words, plants show variation. It was not 
until the middle of the nineteenth century that it was shown that 
living things inherit characters from their parents in a certain 
way and that inheritance in nature obeys fixed laws. Discovery 



of the laws of heredity by Gregor Mendel opened the way to the 
explanation of what man had been able to accomplish in the 
development of improved strains of plants. It has also sim- 
plified the processes of plant breeding and introduced the new 
science of genetics. 

The economic importance of plant improvement to the people 
of the world may be illustrated by improvement in wheat. As 
an example, Roberts of the Kansas State Agricultural College 
made collections of wheat from all parts of the world and especially 
from the wheat-growing regions where conditions are similar to 
those of Kansas. An early winter with little snow killed most of 
the wheat in Kansas. There was one exception. A plot which 
Roberts had planted with seed imported from southern Russia 
passed through the winter without serious harm. The seeds 
were carefully saved and planted, and from this beginning was 
developed the Kanred variety which not only was frost resistant, 
but also ripened earlier than other Kansas wheats, was more 
resistant to stem rust, and the flour of which proved excellent for 

Plant improvement has been of untold benefit to the plant 
grower, who has been able to produce larger and better crops, 
but it has been of even greater benefit to the consumer, who can 
get cereals, fruits, and vegetables of higher quality and at less 
cost than would be possible with unimproved varieties of plants. 

Problem 1. In what ways have plants changed? 

At least 250,000 different species of plants are living on the 
earth at the present time. Rocks are found which contain fossil 
remains of simple plants which lived in the remote past it is 
judged around one thousand million years ago. It is impossible 
to conceive of the time expressed in that figure. The time cov- 
ered by the average life span of a human being is only a fraction 
over a second as compared with the time covered by the history 
of plants as found in the rocks. 

Botanists believe that simple plant life came into existence 
at a much earlier period than that represented by the record of the 
rocks, and that from the beginning many plant forms have come 


and gone, others have remained and become the ancestors of the 
plant life that we see on the earth today. We can ask many 
questions concerning the nature of the first forms of life on the 
earth, but no one has been able to answer definitely any of them. 
They were certainly very simple, probably small bits of jelly- 
like protoplasm possessing the powers of assimilating food, respir- 
ing, growing, excreting wastes, responding to stimuli, and repro- 
ducing. These are the outstanding properties of living matter, 
protoplasm, the most wonderful of the different forms of matter 
of which man has any knowledge. From the very beginning of 
life, there has extended a line or lines without a break living 
material giving rise to more living material. We can trace back 
in our imagination this line from every living thing now on the 
earth through all the countless years to these simple sources. 

The simplest forms of plant life must have lived in the presence 
of water, probably in the sea. They must have been so delicate 
that even the slightest amount of drying would have destroyed 
them. Among the simplest forms of plant life which we know 
at present are the bacteria. It is thought that bacteria in the 
past had an important part in the separation of calcium carbonate 
from the sea-water and in the resultant formation of the great 
deposits of limestone which we find in various parts of the earth. 
It is believed, also, that bacteria have been responsible for the 
laying down of certain deposits of iron ore and of the graphite 
from which the lead in pencils is made. Scientists have estimated 
the time which must have elapsed during the formation of these 
deposits, and from the estimates have been able to guess concern- 
ing the antiquity of such simple plants as bacteria. Whatever the 
forms of early life were, there must have been changes which 
tended to fit these forms to the changing habitats in which they 
lived. As time went on, some appeared which could live on 
land in the moist air of those early periods. From these early 
land plants have developed our mosses and ferns and finally the 
seed plants. As plants progressed, those changes which were 
advantageous tended to add to the chances of survival. Changes 
were not all in a direction which proved an advantage. On this 
account great numbers of plants which once lived on the earth 
have gone out of existence. We have abundant evidence of this 


fact in the fossil remains of plants which are found in the sedi- 
mentary rocks such as limestone, sandstone, and shale. 

We are led, in our discussion of the changes in plants, to the 
two statements: first, that plants of today are descendants of 
plants that preceded them, and these in turn came from other 
plants; second, in time certain forms came to be different from 
their ancestors. We are led to believe that new plants will appear 
on the earth in the future in the form of modifications of plants 
that are now living. 

Problem 2. How do we know that plants have changed? 

Botanists who have made a study of the evidences of change in 
plants in the progress of their development on the earth have 
offered facts to prove to their own satisfaction that the plants of 
the present are modified descendants of the plants which preceded 
them. First, a great variety of plant forms are found as fossils in 
the rocks and coal deposits. Second, the geographical distribution 
of plants as they are found today indicates that many families had 
their origin in early forms in some particular part of the earth's 
surface from which their descendants gradually spread to other 
regions. Third, plant forms of today have a remarkable similarity 
to each other in structure and function of parts. Fourth, the 
plants of today are remarkably similar in their cycle of life. 
Fifth, plant breeders have developed new forms which could 
easily be mistaken for distinct species. Sixth, man has found 
forms in nature (mutants) which are distinctly different from their 

What are fossils? Have you ever gone fossil hunting? If you 
have, you know the thrill one gets upon releasing, from its rock 
prison, a record made millions of years ago when your plant was shut 
off from the light of day by a deposit of clay or sand which subse- 
quently turned to stone deep down in the earth. A blow of your 
hammer brings to light again after all these years what is left to 
show of the plant life which existed in times which are now remote 
geologic history. 

Ancient plants have been preserved as impressions. Some 
part of the plant was covered by clay, sand, or mud, and this 


left a permanent impression in the material which, subjected to 
great pressure and age, turned to stone. 

Much of our knowledge of plants of the past has been gained 
from a study of impressions found in the layers of rock just above 
beds of coal. Fig. 173 shows such an impression of a fern leaf in a 
piece of shale taken from a coal mine in Illinois. It was removed 
from a position in the earth 60 feet below the present surface. 

FIG. 173. Fossil of a part of a frond of a seed-fern. These ferns formed a 
part of the plant life of the Coal Age forests. 

What we know as petrified wood is not in reality wood at all. 
Fig. 174 shows a piece of so-called petrified wood. It has the 
grain and even the cell structure of wood. The original wood 
or plant part was covered with water which contained in solution 
a large amount of mineral matter. This material penetrated the 
wood, and as the wood decayed the mineral matter took its place; 
when the process was complete the rock material had taken the 
form of the original plant material. 



In the earliest rocks few fossils of 
plants are found. There may have 
been many more plants than the 
fossil remains indicate, but during the 
millions of years which followed, the 
rocks were subjected to the effects 
of running water, high temperatures, 
and enormous pressures, any one of 
which conditions could have de- 
stroyed the record. 

Fossils of the different geologic 
periods show that simple plant struc- 
tures were succeeded by structures 
more complex as plant groups suc- 
ceeded one another in the long period 
of their development. The records 
show that great groups of plants de- 
clined and entirely faded from the 
picture. The first seed-bearing plants 
were fern-like. The flowering plants 
as we know them today are a com- 
paratively recent development. In general, the simplest 
plants are found in the oldest fossil-bearing rocks. The 

FIG. 174. Note the resem- 
blance of this "petrified wood" 
to real wood. 

FIG. 175. Ferns and cycads. It is thought that the vegetation in many parts of 
wha tis now temperate America was something similarto this a hundred million years 
ago. (Photograph furnished by the Field Museum of Natural History, Chicago.) 


FIG. 176. Cycas revoluta, the "sago 
palm " of our conservatories. 

fossils of more complex plants are found in the rocks of later 

The first seed plants to attain prominence in the earth's flora 
were all gymnosperms, that is, forms with seeds not enclosed in an 

ovary such as our own living 
cone-bearing trees, like pine and 
spruce. Many of these plants 
became extinct at about the 
time the angiosperms (flowering 
plants with enclosed seeds) were 
becoming established. The few 
forms of these early plants 
which remain are the fern-like 
cycads of the tropical regions 
of both the eastern and western 
hemispheres, and the maiden- 
hair tree (Ginkgo biloba of China) which is the lone survivor of 
what was a large order of trees at about the time the flowering 
plants were becoming numerous. 

Evidences from geographic distribution. Geologists tell us 
that in the course of geologic time the earth has undergone many 
changes which affected plant 
life. During the millions of 
years in which the plant ma- 
terial that later became coal was 
being laid down, the nature of 
the plant life over the earth 
was extremely uniform. During 
this time and, indeed, over 
much of geologic time, the cli- 
mate must have been uniformly 
warm and moist over most of 
the earth's surface. Under 
these conditions plants of the 
same group could have almost world-wide distribution. Be- 
cause of the more uniform conditions in water than on land, 
algae and other water plants of today are more widely distributed 
than species of land plants. It has been noted in a previous unit 

FIG. 177. Dioon, a cycad. Note 

the resemblance to the ferns from 

which the cycads came. 


that, through changes in such plant structures as leaves or flowers, 
plants are able to meet the conditions of a changed environment. 
Plant groups that cannot change are eventually crowded out by 
others more suited to the changed conditions. We probably 
have the explanation in these facts for the entire destruction or 
reduction to a mere vestige of great groups of plants in different 
periods of geologic time. 

Barriers, as oceans, mountain chains, or deserts, have the 
effect of isolating plant groups. It is believed that at different 
times in the history of the earth the land masses were very different 

FIG. 178. Zamia, showing carpellate cones. The cycads, to which this plant 
belongs, are the most primitive of the living gymnosperms. 

in form and extent from the continents as they are at present. 
North America and Asia were once connected by a land bridge. 
There is also evidence that Europe and America were connected 
by land in the north Atlantic, and that, in the south Pacific, 
South America and Australia were more or less connected at a 
very early period. 

At about the time the flowering plants were becoming estab- 
lished as a future dominant group, the Gulf of Mexico extended to 
the Arctic Ocean, forming a barrier between the eastern and west- 


ern parts of the United States. With this invasion by the sea 
here and in other parts of the earth, climatic conditions being 
mild and moist, there was rapid development in plant life. 
In the rocks of this period are found such modern trees as 
oaks and willows, along with many other genera of plants now 

Following this era there was a period of mountain building, 
especially in the western part of the United States, and of land 
elevation, and North America came to have about its present 
form and elevation. With these changes in land contour came 
changes in climate until it was probably about as it is at present. 
The Pacific slope was a region of dry summers and mild wet 

winters, and the eastern part 
of the United States had moist 
summers and severe winters. 
These regions were separated 
by two great barriers, the 
mountains of the west and the 
dry plains to the east of the 

With these changes of con- 
ditions came a sorting out of 
the plants. It is fair to assume 
that the trees of the west slope 
and those of the east slope had 
the same or similar ancestors. 
But because of the differences 
in climate, the vegetation de- 
veloped in different ways. On 
the west slope the trees are mainly conebearing; on the east 
they are mainly deciduous. It is stated that of the nearly one 
hundred trees native to California only two kinds are found east 
of the Sierra Nevadas. 

In general, closely related species of plants are found living in 
the same geographic region, as if the family had its origin in some 
particular center and gradually moved out from that center. 
The cactus family seems to have had its origin in the Mexican 
plateau, whence it spread widely into regions to which it was 

JbiG. JL?y. .Leaf ol the maiden-hair 

tree, or ginkgo, which is one of the 

most ancient of living trees. 


suited. It has been diversified until there are now about 1500 
separate species. No other continent has native cacti. 

Evidence of change from similarity of structures and roles. 

In studying plant cells, it makes little difference what cells we study 
inasmuch as the structure is in general the same. We recognize 
in each a nucleus, cytoplasm, and a wall of cellulose. Some cells 
are more easily studied in the living state than others, so we select 
our living cell from the tissues that are not too complex, or we 
select a single-celled plant. True, some cells are simpler than 
others. A 'bacterium does not have a clearly defined nucleus. 
The nuclear granules are diffused throughout the cytoplasm. 
The bacterium is considered a primitive type of cell. Another 
primitive cell is that of the blue-green algae. 

Any organ or system in the more complex plants might be 
chosen for comparison, and we would find a remarkably similarity 
in structure. Leaves of plants under similar, conditions of light, 
temperature, and moisture are remarkably similar. Likewise, all 
plants which make food make use of sunlight and raw materials 
similarly. Absorption and digestion are accomplished in the 
same general way. Thus comparative anatomy and physiology 
offer many facts which give us reasons for believing that plants 
of the present came from pre-existing forms. 

Evidence from the results of experiments. If we could carry 
on a series of experiments and actually see a new species of plants 
come into existence, the riddle of life would be very much simpli- 
fied. In one way or another, forms of plants in the wild have been 
improved by man for his own use. Some plants, such as wheat 
and apple, have been under cultivation for so long a time that we 
do not know for certain what their wild ancestors were or just 
where the improved forms were developed. The Indians were 
growing maize long before the white man landed in America. 
The potato is also a product of our country, but the form we 
grow is quite different from any other member of its group, the 
nightshade family, which is found growing wild today. 

Numerous varieties of plants have been developed as a result 
of the cultural practices of the practical farmer, gardener, and 
fruit-grower, but not a single entirely new species. Many other 
varieties have been developed through experiments by trained 


biologists, but even trained biologists have not been able to 
produce a new species. 

When we examine the parents of the hundreds of different 
varieties of the cultivated Chrysanthemum and compare the 
parents with their descendants we are struck by the wonderful 
changes that have been brought about in developing, from a 
plant with yellow flowers less than an inch across, to plants more 
than six feet tall and with flowers of many colors and shades eight 
or even ten inches in diameter. Numerous other examples could 
be cited from the abundance of evidence of changes in plants 

FIG. 180. At left, Chrysanthemum indicum, with flowers an inch in diameter, 
one of the ancestors of our showy cultivated chrysanthemum; right, an im- 
proved variety, a descendant of Chrysanthemum indicum. 

brought about in connection with plant propagation and improve- 

Exercise 132. Study any available rocks showing animal or plant remains. 
Find out from your instructor the kind of rock in which your specimen is em- 
bedded. Is your fossil an impression, plant or animal remains, or an infiltra- 
tion of mineral matter taking the place of the specimen? How do geologists 
tell the probable age of fossils? Under what conditions was your fossil formed? 
Try to determine its probable age. Make a drawing of the specimen in your 

Exercise 133. Copy a map in your notebook showing the condition of the 
continent of North America at the time of the ice age. What changes in 
climate brought about the ice age? What changes caused the ice to disappear? 


What changes in the plant life of the continent were brought about by the 
ice age? 

Exercise 134. Using tree books, make a list of trees which are found only 
west of the Cascade Mountains and another list of trees found in the eastern 
part of the United States. Are the same species of trees found in both regions? 
Account for the likenesses or differences in the character of the vegetation in 
the two regions. 

Problem 3. What are the method and cause of change 
in plants? 

The young or progeny of plants tend to resemble their parents. 
We have reason to expect nasturtiums to grow from nasturtium 
seeds and sweet peas from the seeds of sweet peas. Besides, if we 
desire dwarf nasturtiums, we plant seeds from plant parents which 
are dwarf in habit, and if we want pink sweet peas, we select the 
seed from plants which bear pink flowers. The tendency of 
offspring to resemble their parents is known as heredity. 

A plant which results from vegetative reproduction shows a 
remarkable similarity to the plant from which it came. A straw- 
berry sends out runners upon which appear buds, and from these 
buds grow adventitious roots which fasten them to the soil. 
After awhile the bud with roots is able to lead an independent 
life, and the connecting runner from the parent plant withers and 
dies. The young plant, under suitable conditions, will grow into 
a plant similar to its parent in character of leaves, size and flavor 
of fruit, and in fact, similar in every other respect. This is an 
example of vegetative reproduction. In general, the results in 
every other example of vegetative reproduction are similar to 
those related for the strawberry. 

Why are plants which are produced by vegetative means 
similar to the plant which produces them? Every plant starts as 
a single cell. This cell divides and forms two similar cells by the 
process known as simple cell division. In this process there is a 
division of the cytoplasm, and across the dividing cell is formed a 
partition consisting of two walls so that after division each of the 
daughter cells is completely surrounded by a cell wall. But most 
important of all, in cell division there is a division of the nucleus 
of the cell into two exactly similar parts. This division of the 


nucleus has been carefully studied under the high power of the 
microscope. The details of nuclear division can not be studied 
in living cells. Growing structures, as the tip of a young onion 
root, are killed so that the processes of cell division stop imme- 
diately. Then they are prepared for examination under the 
microscope. Careful staining brings out the structures so that 
they can be studied. 

Cell division. Scattered through the nuclear protoplasm and 
arranged in a sort of net are granules of material which stain more 
deeply than any other parts of the nucleus. The material which 

makes up these granules is 
known as chromatin. When a 
stained lengthwise section of a 
young root tip is examined, 
most of the cells are seen to 
be in the resting condition with 
cell wall, cytoplasm, and the 
nucleus in which are the 
chromatin granules plainly 
showing. A careful study of 
the section reveals some cells 
in which the chromatin gran- 
ules are no longer in the net 
arrangement, but have taken 
the form of a dense coiled ribbon 
the nucleus. It has been 

FIG. 181. Cells near the tip of an 
onion root, showing in three cells the 
darkly stained chromosomes. These 
three cells are in a state of division, 
whereas the remainder of the cells are 


found that this is not a single 
ribbon but rather one with a 
division along the middle 
throughout its length. This double ribbon is recognized as a be- 
ginning stage in simple cell division and is known as the spireme. 
Looking further among the cells of our preparation we see that 
in certain cells the spireme has broken up into a number of sections 
and that each section has divided into two exactly similar parts. 
The sections are of various shapes and are arranged in a plane 
across the cell midway between two sides. 

Examining still other cells, we find a peculiar behavior. It is 
just as if an elastic thread were attached to each section and with 


FIG. 182. Stages in cell division. (The nuclei redrawn from a figure in Sharp's 
Introduction to Cytology, 3rd Edition. McGraw-Hill Book Co.) 


the other end of the thread attached near the ends of the cell (the 
poles of the cell), half of the threads attached to one pole and half 
to the other. Cytologists (persons who study cells) have found 
that one section of a pair seems to be attracted or drawn to one 
pole and the other section of the pair to the other pole. Some 
force causes the two members of each pair of sections to separate 
and move towards the opposite ends of the cell. Some cells of the 
preparation will show the sections in an aggregation at the two 
poles of the cell, and since each group contains a section from each 
pair the two groups will be exactly similar in number and kinds of 
sections. These take a deep stain and for this reason have been 
named chromosomes. 

Other cells of our preparation will show cell walls forming 
across the equator of the mother cell, the chromosomes breaking 
up into chromatin granules and the cell division nearing comple- 
tion, after which the two daughter cells may be called resting cells. 
We now have two similar cells which have resulted from a division 
of material contained in the mother cell. 

What are chromosomes? Biologists who have made a study 
of heredity are of the opinion that characters are handed down 
from parent to offspring by means of the chromosomes of cells. 
Small bodies of material are contained within the chromosomes, 
and these carry something which determines the characters of the 
new individual. These bodies are known as genes or determiners. 
Each chromosome seems to be made up of a large number of genes 
or determiners, and when a chromosome divides in cell division 
the process includes a division of the determiners so that the two 
daughter cells will have exactly similar determiners. That is 
why the daughter cells are similar to each other and also similar 
to the mother cell from which they came. 

When a new plant develops from a portion of stem, leaf, or 
root of another plant by vegetative reproduction, the determiners 
in the cells of the new plant are exactly similar to those of the 
parent plant. Though the surroundings may have the effect of 
modifying the form and size of the offspring, in general the 
young plants tend to be similar to the plant from which they 
came. They are really a part of that plant set over into a new 


Exercise 135. Why do the two daughter cells resulting from simple cell 
division have similar characteristics? In this exercise you will see cells in the 
resting condition and different cells showing various stages in simple cell divi- 
sion (mitosis). It is not easy to find and study the progress of cell division 
under the high power of the microscope, but it is necessary to know what takes 
place in a dividing cell in order to understand inheritance. So, with the help 
of your teacher and by referring to models of dividing cells or to the drawings 
in your book, you will be able to identify dividing cells in the prepared slides. 

Examine, under the low power of the microscope and then under the high 
power, a lengthwise section of an onion root tip stained to show cell structure. 
Look for resting cells and cells in the process of division. In general, how are 
the dividing cells different from the resting cells? Why is the root tip a good 
place to find dividing cells? What is the result of cell division in the root tip? 

Read the description of cell division given in the preceding pages, and 
compare the description with what you see under the microscope. 

Make a series of six enlarged drawings or diagrams of cells as follows: 

1. Resting cell, showing: 

a. Cell wall. 

b. Nucleus. 

c. Chromatin network. 

2. A cell showing a nucleus in which the chromatin network has become 
arranged in a continuous looped band or spireme. 

3. A cell showing separation of the spireme into definite pieces or chromo- 

4. A cell showing the chromosomes arranged in a plate across the middle. 

5. A cell showing the half -chromosomes moving to opposite poles of the cell. 

6. Formation of cell wall between two daughter nuclei resulting in two new 

The cell of the onion root tip has been found to have sixteen chromosomes. 
How many chromosomes has each of the daughter cells? What is the source 
of the chromatin material in the daughter cells? Remembering that chromo- 
somes are thought to be bearers of determiners of characters, explain: 

1 . Why the daughter cells are similar to the cell from which they came. 

2. Why the daughter cells are similar to each other. 

3. Why the vegetative cells of a plant that started from a single cell (the 
fertilized egg) bear the same hereditary characters. 

4. Why the vegetative cells of plants which are started from cuttings bear 
the same hereditary characters. 

What is the inheritance of plants that have two parents? 

In a plant that is the product of sexual or gametic reproduction 
two lines of heredity are represented. You will remember that 
in gametic reproduction of flowering plants the sperm or male 
gamete, produced in the germinating pollen, unites with the egg 


or female gamete, which develops within the ovule, to form the 
zygote. Thus the chromosomes from the sperm cell together 
with the chromosomes from the egg cell are the chromosomes of 
the zygote. You can see that the zygote, then, carries chromo- 
somes with their genes which came from the plant which produced 
the sperm and also chromosomes with their genes which came 
from the plant which produced the egg. The young plant which 
develops from the zygote will be made up of cells each of which 
has chromosomes with their genes which came from both of the 

The vegetative cells of plants of any species all contain the 
same number of chromosomes. Each sex cell or gamete contains 
half as many chromosomes as are found in a vegetative cell or 
zygote. The chromosome number is halved in a peculiar kind of 
division in the process of formation of the gametes, and moreover, 
there is an assortment of the hereditary determiners. 

When a zygote is formed from fertilization of an egg of one 
plant by a sperm of a plant of another strain or species, the plant 
which results is called a hybrid. In cross-fertilization there is a 
combination of determiners. As a result many different kinds of 
hybrid plants may come from the same parents. Thus another 
way in which plants change results from cross-fertilization. This 
is known as hybridization. 

The combination of characters which a plant inherits from its 
parents may be such as will make the plant less fit to meet the 
conditions of the surroundings than either of the parents. In 
this case the plant will not survive in competition with other 
plants. On the other hand, if a plant inherits the strong char- 
acters instead of weak characters from both parents, then the 
plant will succeed and leave offspring which may crowd out 
plants of other strains which are not so well fitted to the environ- 
ment. In this way new strains of plants are coming into existence 
and old strains are disappearing. 

Why do offspring from hybrid parent plants differ? About 
the middle of the last century, Gregor Mendel, an Austrian 
monk, learned much about heredity by experimenting with com- 
mon plants in his garden. In one experiment he planted wrinkled 
peas and smooth peas and was careful to give both the same kind 



of growing conditions. When they were in bloom he removed the 
anthers from the flowers of the one set of plants, say the ones from 
round seeds, before any pollen was ripe. At the proper time he 
transferred pollen from the plants from wrinkled seeds to the 
stigma of the flowers without anthers. This was artificial 
cross-pollination. The result was cross-fertilization. The sperm 

FIG. 183. Diagrams illustrating Mendel's laws of heredity, (w) indicates 
presence of the determiner for the recessive character, wrinkled; and (R) 
indicates the presence of the determiner for the dominant character, round. 
In the Fi generation one of the parents is pure for the character, wrinkled, 
that is, it carries no determiners for pea shape except those for wrinkled; in 
like manner, the other parent is pure for the character, round, as it carries 
only determiners for the round shape. The results of the cross between these 
two plants is a plant which is a hybrid, that is, it carries determiners for both 
wrinkledness and roundness. Study the diagrams for the F 2 and Fg genera- 
tions as you read the discussion of Mendel's Laws in the context. 


carried the determiner for wrinkled to the egg which possessed the 
determiner for round, and the seed which developed carried the 
two determiners for the two characters wrinkled (w) and round (R). 
The character of the seed resulting from the cross may be repre- 
sented by wR (hybrid), which indicates that both determiners are 

Mendel found that the hybrid offspring of pure wrinkled (ww) 
and pure round (RR) parents were all round. He planted these 
round peas and found that in this second (F%) generation both 
wrinkled and round peas were produced, and further that three- 
fourths of the peas were round and one-fourth were wrinkled. 
He planted some of the F% wrinkled peas and self-pollinated the 
flowers. Only wrinkled peas developed on these plants. He also 
planted Fz round peas and self-pollinated the flowers. In this 
case one-third of the plants produced only round peas and two- 
thirds of the plants produced both wrinkled and round peas as in 
the Fz generation. These results may be illustrated by diagrams: 

Mendel's laws. As a result of this experiment and many 
similar ones, Mendel came to the conclusion that characters are 
handed down from parents to offspring in a perfectly definite way. 

1. Unit characters. Mendel thought of the plant as made up 
of distinct and separate characters, as dwarf ness, white petals, 
and smooth seed coat. He called these unit characters. In the 
F2 generation when wrinkled peas are crossed with round peas, 
according to our modern way of thinking, cells of the wrinkled 
pea plants contain a double set of determiners of the wrinkled 
unit characters or (ww) and their gametes would contain only 
one or (w). When the wrinkled-round (wR) peas of this same F% 
generation (see Fig. 183) are planted, the hybrid plants produce 
two kinds of sperm cells, one having determiners of the wrinkled 
(w) unit character and the other the round (R) unit character. 
In like manner, corresponding types of eggs would be produced. 
The different combinations of eggs and sperms are shown in Fig. 183. 

In this case one of the zygotes (ww) will produce a plant with 
only the determiners for wrinkled unit characters in the cells, a 
second zygote (RR) with only those for the round unit characters, 
and the other two zygotefc (wR) will have determiners for both 
the wrinkled (w) and rouncl (R) unit characters. 


2. Segregation. In the F^ generation shown in Fig. 183 the 
two (wR) peas are round, but when the plants from these peas 
produce gametes, two kinds (w) and (JR) are produced, that is, 
the unit characters are segregated in the production of gametes. 

3. Dominance. Referring again to Fig. 183 it is seen that in 
the F% generation three-fourths of the peas are round, although 
the determiners for the character wrinkled are present in two- 
thirds of these round peas. For some reason the wrinkled char- 
acter, although its determiners are present, does not show in the 

FIG. 184. Corn plants showing the Mendelian three- to-one ratio. The seed 
from which these corn plants grew produced green plants and albino plants 
without any green color in the Mendelian ratio, albinism being a recessive 


outward appearance of the seed when the round character is 
present. Mendel called the kind of character represented by 
round, dominant, and the wrinkled character, recessive. Other 
dominant characters with their recessives in garden peas are 
yellow seeds green seeds, colored flowers white flowers, and 
tall vines dwarf vines. 

Exercise 136. How do plants inherit the characters of their parents? 
Corn is suitable material for use in experiments in heredity similar to those of 


Mendel. Material for these experiments can be procured from Mr. George S. 
Carter, Clinton, Conn., and from other dealers in biological supplies. 

A. Inheritance of starchiness and sweetness in corn 

1. Examine a hybrid FI ear of corn resulting from a cross between pure sweet 
corn and pure starchy corn. 

2. Also note the appearance of the ears of the parent stocks. Which is 
dominant starchiness or sweetness in corn? 

3. Examine an ear of corn grown from seed of an FI hybrid plant. Count the 
starchy grains and the sweet grains. What is the ratio of starchy to sweet? 

4. Make a diagram to show the FI generation and another to show the 
F 2 generation. (See Fig. 183.) 

5. If you planted the sweet grains, how many different kinds of grains 
would you get? 

6. If you planted the starchy grains, how many kinds of grains would you 

7. Which would be easier to pick out in pure state, starchy grains or sweet 

8. How does this corn illustrate Mendel's Law of Dominance? The Law of 
Unit Characters? The Law of Segregation? 

B. A strain of corn has been developed which carries the albino character 
as a recessive. That is, in plants from the seed of this strain a part of the 
plants will appear without chlorophyll. Can a plant without chlorophyll 
grow to maturity and produce pure albino seed? Explain. Representing the 
green character by G and the albino character by w, the so-called hybrid plant 
carrying the determiner for albino would be represented by Gw. 

1. How many different kinds of sperms and how many different kinds of 
eggs does this plant produce? 

2. Make a diagram similar to that in Fig. 183 for this corn, showing 
what occurs when it is self-fertilized. How many different kinds of seed are 
produced when the zygotes mature? 

3. Plant 16 grains of the Gw seed in a pot and determine the ratio of green 
plants to white plants. 

Exercise 137. Why is the Mendelian ratio 1 :2 :1? It was noted above that 
in Fz of the cross between wrinkled peas and round peas the ratio of the dif- 
ferent forms of the offspring was 1 RR: 2 Rw: I ww. This results from the 
chance meeting in fertilization of the sperms, R and w, with the eggs, R and w. 
This may be demonstrated by a simple experiment. Mix in a can 100 red 
beans and 100 white beans of about the same size. Without looking, remove two 
beans at a time and place in separate piles the pairs of red beans, the pairs of 
white beans, and the pairs of red and white beans. When you have removed all 
the beans from the can, count the number of pairs in each pile. What is the 
ratio of numbers in each pile? This represents the chance combinations of 
determiners which result from cross-fertilization. By the law of chance the 
determiner for red, say of a sperm, is combined with the determiner for red in 
an egg; the determiner for red in a sperm is combined with the determiner for 



white in an egg; and the determiner for white in a sperm will be combined 
with the determiner for white in an egg. If we let R stand for red and w for 
white, then the character of the resulting zygotes will be indicated by RR, 
Rw, and ww. 

Construct a diagram to show 
the chance combinations of this 
experiment. (Fig. 183.) 

How do new forms of 
plants originate? In study- 
ing heredity in hybrid plants 
and their offspring we found 
that determiners for char- 
acters are handed down to 
offspring in a perfectly defi- 
nite way. In all our dis- 
cussion with regard to 
change in plants due to 
hybridization we have not 
accounted for the appear- 
ance of new characters 
which were not present in 

the ancestry in any visi- FlG - !85. Different types of leaves on the 
U1 f T j.u i*/m same branch of scarlet oak. The lower 

bleform. In the year 1699 shaded leaves are mesophy tic; those above 
there was introduced into in the sun are xerophytic. These are modi- 
Holland and England from fications due to environment and not to 
Sicily a sweet pea species, heredity. 

Lathyrus odoratus, the flowers of which were purple and blue 
in color. In the descendants of this plant there appeared, in 
1718, a plant which produced white flowers and in 1731 another 
I which produced flowers which were pink and white. In the 
generations which followed, the seeds from the white-flowering 
plants continued to produce plants which bore only white flowers 
and the seeds from the pink-and- white flowering plants, produced 
plants with only pink-and-white flowers. From this original plant 
species there have been produced hundreds of varieties of sweet 
peas bearing flowers differing in size, form, and color, and in 
each case the offspring bears flowers like those of its parent; that 
is, the plants breed true. 


Any variation in plants, such as those in color, size, and form 
in sweet peas, which passes from parent to offspring unchanged is 
known as a heritable variation. Dwarfness in the midget sweet 
pea and in the low-growing nasturtiums is a heritable variation. 
Variations due to differences in the conditions of the environment, 
as differences in size and shape of oak leaves on the same tree, 
are known as fluctuations. These variations are not inherited. 

, FIG. 186. The weeping birch is a mutation. 

The white sweet pea which appeared from purple-and-blue 
stock in 1718 and the pink-and-white one that appeared in 1731 
from the same parent stock are examples of mutants or mutations. 
Mutation has furnished many plants of value to man. From the 
small currant tomato or love apple of a hundred years ago have 
developed by mutation many of the varieties of tomato which we 
know today, some of which produce fruit weighing more than a 

Sometimes a branch has appeared which produced fruit or 
foliage differing in marked degree from that of the main plant. 
In this way the Boston fern and its many varieties have appeared. 



All the improved varieties of the seedless orange have originated 

from buds. These variations are known as bud gutatipns.^ 

There are reasons for believ- 
ing that many mutations are tak- 
ing place in nature. Most of these 
changes are probably of no ad- 
vantage to the plant, or they may 
even prove a disadvantage. These 
new forms may not be able to 
compete successfully with other 
plants of the environment and as 
a result soon go out of existence. 
On the other hand, a mutation 
may be better fitted than its 
parents to meet the conditions of 
the environment so that in the 
struggle for existence it succeeds 
and may eventually crowd out 
other plants. This principle is 
known as natural selection. Mu- 
tations appear, and they succeed 
or fail according to whether or 
not they are able to compete with 
other plants in their habitat. 

FIG. 187. -The Foray thia branch 
on the right shows the regular 
Forsythia habit of bearing two 
opposite leaves at a node. The 
branch on the left is a bud muta- 
tion from the same plant bearing 
three leaves at each node. 

Exercise 138. How do fluctuating characters differ from hereditary 
characters? 1. Compare leaves of mulberry which have grown in the sun with 
leaves from the same tree which were shaded. Note especially size and shape 
of leaves and their thickness and texture. Make drawings to show differences 
in size and shape. How do the sun leaves of the mulberry differ from shade 
leaves? How do you account for the differences? 

2. Examine pine needles and leaves of the rubber plant, a xerophytic 
grass, and Sedum or live-forever. Note especially the thickness, leaf surface, 
and texture of a number of leaves of each kind. Does there seem to be any 
marked variation between the different pine needles, between the different 
rubber-plant leaves, between the different grass leaves, or between the different 
succulent leaves of Sedum? Characters like the ones here noted that are 
handed down from parent to offspring are known as heritable characters. 
Any marked change which is heritable is known as a mutation. 

Exercise 139. How do new forms of plants originate? A. The common 
cabbage and a number of other varieties of plants have developed from the 


FIG. 188. Two heads of the red sunflower (at sides) and a head of the ordinary 

yellow sunflower (center). The red sunflower is a mutation. (Photograph 

furnished by T. D. A. Cockerell.) 

wild cabbage, a native of Europe. From seed catalogues select a list of 
plants as Brussels sprouts, cauliflower, etc., which have come, or seem likely 

to have come, from the wild form of cabbage. 

What part of the plant, as stem, root, flower, 

is useful to man in each one of the varieties 


B. Make a comparison of characteristics 
of different varieties of chrysanthemums listed 
in a flower catalogue of some prominent seeds- 
man. In what ways do these vary from the 
Chrysanthemum indicum, one of the original 
parents. See Fig. 180. 

C. Make a list of vegetables and another 
of flowers designated as hybrids in the seed 

D. Make lists of plants which are prop- 
agated vegetatively. Which are more apt 
to come true, plants from seeds or plants 
from some vegetative part as a tuber or 

FIG. 189. Chrysanthemum 
plant showing bud muta- 
tion. This plant of a white- 
flowering variety produced 
a branch which bore pink 


Problem 4. How does man develop new kinds of plants? 

Many of our most widely cultivated plants have been in the 
process of development for so many centuries that we know little 
of their wild ancestors. A small-grained variety of wheat was 
found among the remains of the lake dwellers in Switzerland which 
dates from the early stone age. 

How have common crop plants originated? Indian corn or 
maize is a New World product. When the white man first visited 
America he found the Indians making use of maize as a crop plant. 

FIG. 190. The wild potato of southwest United States (Solanum jamesii). 
(After Fitch, Colo. Agr. Exp. Sta., from Robbins, in Botany of Crop Plants.) 

We know of no wild form which can be recognized as the immediate 
ancestor of this important crop plant. 

The cultivation of the apple dates back at least as far as that 
of wheat. The varieties which were used in very early days were 
small, measuring an inch to an inch and a quarter in diameter. 
This wonderful fruit seems to be a native of southeastern Europe. 
The apple was widely distributed in Europe in its uncultivated 
state, but the fruit was much inferior to that of the cultivated 
varieties of the present. There are at least five native crabapples 
in America. These wild apples are fit only for jelly-making and 
for cooking. The Indians made some use of the wild apple, and 
when the white man introduced the common apple, they were 


quick to take advantage of it. Remains of old Indian orchards 
still exist in different parts of the country. 

You have all enjoyed the luscious fruit of the peach. Have 
you tasted the fruit of the nectarine? An interesting riddle is con- 
nected with the history of these two' fruits. The peach is readily 
recognized by its well-known characteristics, including a downy 
covering. A peach tree may be producing crops of peaches year 
after year and all of a sudden it may begin to produce nectarines, 
similar to the peach but without the downy covering. Afterwards 
the tree or a part of it may produce either peaches or nectarines. 
The nectarine is known as a bud sport or bud mutation of the 
peach. Bud mutations have been responsible for the appearance 
of some valuable varieties of plants which have been retained by 
man and propagated by vegetative methods. 

The orange belongs to the group of citrus fruits. The wild 
variety produces fruits which are bitter. The sweet strains have 
been long in cultivation. The seedless orange arose as a bud muta- 
tion; it has been maintained by vegetative propagation and im- 
proved by that method and by selection. 

The grape of our western coast came from Old World stock; 
that of eastern United States originated from native species of 
wild grapes. The more than fifteen hundred varieties of European 
grapes have all descended from a single species, Vitis vinifera, 
supposed to be native of Asia. The early settlers of America 
made persistent attempts to establish vineyards of the European 
grapes in the new country, but without success. The failures 
were due to an insect pest, Phylloxera, a plant louse which infests 
the roots and to which the European grape was especially sus- 
ceptible. This pest was later introduced on some nursery stock 
into Europe where it played havoc with the vineyards. The prob- 
lem was finally solved. Someone noticed that certain native 
American species were almost immune to the attack of the phyllox- 
era. Grape vines were started from pieces of vine of these 
immune plants, then the European Vitis vinifera was grafted on 
these plants. The grapes grown on our Pacific coast are varieties 
developed from Vitis vinifera. Is this method of producing im- 
mune grapes vegetative or sexual? 

The grapes of the eastern part of the United States have all 



been developed from native American species. The original 
Concord grape vine is still growing in Concord, Massachusetts. 
It developed as a chance seedling from the native Labrusca species, 
the northern fox grape. Was the method used in producing the 
Concord grape vegetative or sexual? 

How are desirable characters of parent plants combined in 
offspring? In our discussion of Mendel's law only one set of con- 
trasting characters was considered in the example taken from the 






FIG. 191. Diagram showing the results of self -pollinating a pea plant which 
carries the determiners for two sets of contrasting characters, in this case, 
yellow and green, and round and wrifikled (YgRw)} or of crossing two of the 
plants. Zygotes 1-9 will produce yellow, round peas, since determiners for the 
dominant characters, yellow and round, are present in each; 10-12 will pro- 
duce yellow wrinkled peas, as the determiner for dominant yellow is present, 
that for dominant round is not present, and that for recessive, wrinkled is 
present. Why will 13-15 produce green, round peas, and 16 green, wrinkled 



experiment with peas. These characters were round and wrinkled. 
If now we take a second pair of contrasting characters into our 
experiment the situation becomes much more complex. 

The characters actually chosen by Mendel in one set of experi- 
ments with peas were yellow-round and green-wrinkled; that is, 
one of the parent plants selected produced peas which were yellow 
and round, and the other plant produced peas all of which were green 
and wrinkled (Fig. 191.) In this case when the plants were cross- 
pollinated, one set of gametes, say the sperms, carried determiners 
for yellow and round and the other set of gametes, the eggs, car- 
ried the determiners for green and wrinkled. The zygote which 
resulted from the union of these two gametes contained the de- 
terminers for yellow, round, green, and wrinkled. Since yellow is 
dominant over green, and round over wrinkled, the peas of this FI 
generation were all yellow and round in character. 

In Mendel's experiment the seeds of this FI generation were 
planted and the cells of the hybrid plants which resulted all con- 
tained the determiners for the characters yellow, round, green, 
and wrinkled, and when these plants produced gametes, combina- 
tions of determiners present were as follows : YR, Yw, gR, and gw. 
The table (Fig. 191) shows the different possible zygotes. Since 
yellow and round are dominant over green and wrinkled, these 
characters, when their determiners are present, will show in the 
peas produced regardless of whether or not the determiners for 
green and wrinkled are present. 

It is seen, then, that in general, out of every sixteen dihybrids 
resulting from this cross, nine were yellow and round, three green 
and round, three yellow and wrinkled, and one green and wrinkled. 
The characters of the parents for color and nature of seed coat 
were combined in different ways in different individuals. The 
keen eye of the practical plant breeder is able to select from thou- 
sands of hybrids the individual plant which may have the desired 
combination of characters. 

If the selected specimen is a potato or fruit or other plant which 
may be propagated by vegetative means, the offspring will all 
have the desired combination of characters. If, on the other 
hand, the plant can be propagated only from seed, certain offspring 
of the dihybrid will show the desired combination of characters, 



but other combinations of characters will also appear in others of 
the offspring. 

Suppose we desire the combination, yellow-wrinkled in our 
plant. We select only yellow wrinkled seeds. Seeds YwYw, of 
course, will produce only plants which bear yellow wrinkled peas. 




FIG. 192. Results of self-pollination of a plant from the YwYw zygote. All 
of the offspring are pure yellow wrinkled like the parent. 

But some of the yellow wrinkled seeds are of the Ywgw kind. 

If the Ywgw seeds are planted, some of the peas which result 
will be green-wrinkled. However, by continued selection and 
isolation that is, by pedigree culture it is often possible to 
secure dihybrids which will breed true with respect to the desired 
combination of characters. 






FIG. 193. Results of self-pollination of a plant or crossing two plants from 

the Ywgw zygotes. J of the offspring are pure yellow wrinkled (YwYw); 

\ are pure green wrinkled (gwgw); and ^ are yellow wrinkled (Ywgu), but, 

like the parent, are not pure. 

In the same way trihybrids may have the combination of three 
desired characters from parents which differ from one another in 
three respects. 

The highly developed technique of Burbank made it possible 
for him to produce a trihybrid, Burbank's shasta daisy, in which 
he secured a combination of characters belonging to three distinct 


parent stocks, American, English, and Japanese. The American 
daisy, or marguerite, is a very free bloomer, but it has a sprawling 
habit and unattractive leaves. The English plant is attractive 
and sturdy and with handsome leaves. The Japanese type has 
attractive lustrous flowers. Burbank's attempt to combine in 
one plant all the desirable features of the three resulted in the 
shasta daisy. 

What methods has man used in the improvement of plants? 
The most common method of plant improvement is the continuous 
selection of the best for seed. This can be illustrated by the 
practice of any practical corn-grower who goes into the field of 
mature corn and picks seed for the next crop. He selects for 
certain characters, as desirable stalk growth, long kernels, ears 
well filled out to the tip, and earliness of maturity. In this way 
an artificial selection of the best is made generation after genera- 
tion. This method, known as mass selection, is effective in 
improvement, but not for originating new forms. 

The more recent method of plant improvement was that 
which was introduced by Gregor Mendel in 1865 and which came 
into general use in the study of heredity and plant improvement 
thirty years later as the science of genetics. This method is known 
as pedigree culture. Whereas mass selection secures a good 
average plant, pedigree culture is concerned with the selection 
of the individual plant with a view to the development of a 
superior plant strain. This is the method' used in the develop- 
ment of strains of plants which are drought-resistant or disease- 
resistant or resistant to damage by frost. 

Thirty-five years ago, a very destructive chestnut bark disease 
was introduced into America on nursery stock. The American 
chestnut is threatened with extinction from the ravages of this 
disease, as it is especially susceptible. It has been found that a 
Japanese species and also a Chinese species of chestnut are resistant 
to the attack of the disease. Since the European as well as the 
American species of chestnut is susceptible, the future of the 
chestnut in the world rests with the use and improvement of the 
resistant strains by hydridization. In fact, a resistant strain 
has been produced already by crossing the American bush Chin- 
quapin with the Japanese variety of chestnut. 



Pedigree culture is illustrated also by the development of a 
watermelon resistant to the destructive wilt disease. The citron 
Citrullus vulgaris is immune to wilt but inedible. The Eden 
watermelon is edible, but susceptible to wilt. The two were 
crossed, producing in the first generation hybrids of wonderful 
vigor and productiveness. The second generation was extremely 
variable, the citron characters appearing to be dominant. From 
4000 plants, ten fruits were selected. Seeds of these were planted, 

FIG. 194. Artificial pollination. The flowers to be cross-pollinated are emas- 
culated, and then kept covered with a bag until the pistils are pollinated and 
the stigmas have passed the receptive stage. (From Division of Pomology, 
College of Agriculture, University of California.) 

and after five years of selection the desired melon, combining wilt 
resistance and edibility, was secured. 

The characteristic noted in the F\ hybrid melons is known as 
hybrid vigor. Frequently the hybrids resulting from a cross 
between two strains show marked increase in vigor and pro- 
ductiveness over those of either parent. Burbank made use of 
the phenomenon of hybrid vigor in the production of a new walnut 


tree by crossing the English walnut with the California black 
walnut. The hybrid which resulted grew in 14 years to be 75 feet 
tall and to have a trunk 2 feet in diameter while a tree of the 
black walnut parent type grew a trunk only 6 or 8 inches in 
diameter and attained a height of only 20 feet in 31 years. 

Suggested activities. 1. Write a summary of the history of some fruit, as 
the Delicious or Jonathan apple. 

2. Write a revitw of a book describing the work of Luther Burbank. 

3. Write a paper on the importance of the work of Gregor Mendel. 


1. What reasons have we for thinking that the earliest forms of life on the 
earth were plants? 

2. Why is it more likely that the earliest plants lived in water than that 
they lived on land? 

3. In what ways did plants change in form in changing from a water habitat 
to a land habitat? 

4. What is a fossil plant? 

5. In what different ways are fossils formed? 

6. It is known that limestone was formed at the bottom of the sea. How 
can you explain the fact that great limestone deposits are found at present far 

7. If you have visited a " petrified forest " prepare a report to be read to 
the class describing what you saw. 

8. Under what conditions could mineral deposits take the place of the wood 
and form the so-called petrified wood? 

9. How do you explain the fact that we find fossils of plants which are 
different from any plants now living? 

10. How can you explain the fact that fossils of tropical plants are found 
in regions which now have a temperate climate? 

11. Explain the fact that vast coal deposits are found in Alaska. 

12. Which plant species are more likely to change with a changing environ- 
ment, those which reproduce only by vegetative methods or those which 
reproduce sexually? Explain. 

13. Explain why the two daughter cells resulting from simple cell division 
are similar and also similar to the mother cell. 

14. How does a knowledge of Mendel's Laws of Heredity Jielp man in pro- 
ducing improved varieties of plants? 

15. How does a farmer use mass selection hi selecting seed corn from his 


We learned in the last unit that during past geologic times 
the character of the plant life of the earth has changed; that in 
the course of time certain species of plants have become extinct; 
that new species have come into existence; that there has been 
constant change. Scientists are convinced that the present is 
the offspring of the past; that the plants which populate the 
earth today have characteristics like those of the past, because 
they are offspring of plants of the past. But it appears that, 
although there has been increasing change for centuries and cen- 
turies, certain plants in existence today resemble very remarkably 
those of early geologic times. We refer particularly to such 
aquatic plants as the algae (pond scums, seaweeds). Undoubt- 
edly, the reason for this lack of change of such plants is that water 
is a uniform environment, subject to very little change in tempera- 
ture, in oxygen and carbon dioxide content, and in its mineral 
composition. Water of the earth has been very much the same 
throughout geologic history. Consequently, the plants of water 
have not undergone much change during that time. Land environ- 
ments, on the other hand, are extremely variable. Different types 
of soil, varying in texture, in chemical composition, in exposure 
to the sun's rays, in temperature, in water-holding capacity, pre- 
sent different sets of conditions under which plants must grow. 
And we find that the kinds of plants which grow under these dif- 
ferent conditions are quite dissimilar. We are forced to the con- 
clusion that differences in environmental conditions have been 
responsible for inducing the changes which have occurred in 
plants. So today we find on the earth's surface a great assemblage 
of plants differing greatly in their form and structure and habits 
of growth, populating all types of environments salt water, fresh 



water, cold water, hot water, sandy soils, clay soils, lime soils, 
humus soils, wet soils, dry soils, rocky exposures, arctic and alpine 
regions, tropics, and so on. And these plants grow under the 
particular set of conditions they do because they have structures 
and habits of growth which adapt them to these conditions. 

There is strong evidence that life as we know it on this earth 
originated in the water. The first forms of animals and plants 
were water forms. Water plants very similar, probably, to cer- 
tain simple algae found in the waters of the earth today were the 
ancestors of our present-day plants, both of the water and of the 
land. As the surface of the earth changed, as climates changed, 
new kinds of plants came into being. But these new kinds re- 
sembled their parents; and, too, they differed from their parents. 
Slight modifications in form or behavior may have enabled certain 
offspring to live and reproduce under slightly different conditions 
from those to which their parents were accustomed. After many, 
many generations the individuals may come to be very unlike 
their ancestors of ages past. 

All plants have much in common. This is one reason we 
regard them as related. Pond scums, molds, mildews, mosses, 
ferns, pines, roses, potatoes, oaks, and wheat all have certain 
very definite characteristics which indicate a common ancestry. 

Their cell structure is the same; the living stuff, protoplasm, 
is essentially similar; they respire, nourish their bodies with the 
same kinds of food, digest food, and reproduce in very much the 
same manner. Living things which have so many likenesses in 
their fundamental structures and processes, even though they may 
look unlike, must certainly be related. 

Problem 1. How are plants classified? 

There are some 250,000 different kinds of plants in the world. 
We call these different kinds species. We recognize different 
degrees of relationship among them. For example, we recognize 
that different kinds of oaks are more closely related to each other 
than are oaks to maples. It is likely, we would assume, that 
apples and pears are closer " kinfolks " than are apples and wheat. 
And it would be safe to say that the different kinds of flowering 


plants are more closely allied than are flowering plants and mush- 

From the earliest times man has attempted to classify plants, 
just as he has attempted to classify all sorts of things, even his 
ideas. One of the early attempts to classify the common plants 
about us, before the days of the microscope and the detailed 
knowledge of plant structure, was based upon habit of growth. 
This was a classification into three large groups, namely trees, 
shrubs, and herbs. This was a sort of artificial or arbitrary 
system of classification. Study revealed the fact that bamboo 
(a tree) and corn (an herb) were really more closely related than 
bamboo and cotton wood, both trees; and, as another example, 
the strawberry (an herb) is more closely allied to the rose (a shrub) 
than are roses and sagebrush (both shrubs). The reader will 
readily think of other examples. An attempt has been made in 
grouping plants to place together those with fundamental like- 
nesses; with similarities in structure which represent true rela- 
tionships. The student readily understands that frogs, grass- 
hoppers, and kangaroos are not placed in the same natural group 
just because hopping about is common to them. The body form, 
methods of reproduction, and all life habits are very greatly 
different. But frogs and toads are closely related; kangaroos 
and opossums are closely allied; and grasshoppers and locusts 
are very much alike. 

What then are the characters, among plants, which express 
true affinities? How do we tell that one plant is near in its rela- 
tionship to another? Is it the kind of root system they have? 
Is it the sort of stem they have? Is it the kind of environment in 
which they live? None of these. Scientists have learned to 
place reliance in reproductive structures and behavior as marks 
of true relationship. For example, beans, peas, clover, alfalfa, 
and peanuts are naturally grouped together because the flowers 
(reproductive structures) are built on the same general plan; 
goldenrods, chrysanthemums, daisies, sunflowers, and thistles are 
grouped together also because of very similar floral structure. 

Space will not permit a discussion of the various systems of 
classification of plants which have been proposed during the last 
several centuries. Suffice it to say that one system has replaced 


another as new facts about the plants of the world have come to 
light; and that system is considered most perfect which most 
accurately expresses the true affinities of plants. 

Problem 2. What are the four great groups of plants? 

One modern system of classification which has been widely 
accepted would throw all plants of the world into four large groups, 
called phyla or grand divisions. These four large groups are as 

1. Thallus plants (thallophytes), such as the pond scums, 
.seaweeds, bacteria, molds, mildews, yeasts, rusts, smuts, mush- 
rooms, and toadstools. The plants belonging to this large group 
have no roots, stems, or leaves in the ordinary sense; they have 
no flowers, the reproductive organs and methods of reproduction 
being very simple. Certain thallus plants are regarded as simple 
and primitive, that is, the first kind of plants to appear on the 
earth; in fact, they are believed to be the progenitors of more 
complex plants. 

2. Mosses and liverworts (bryophytes). These are well- 
known plants growing in moist places. Usually, they are close to 
the soil, that is, in contact with a source of water. No part of the 
plant is very far away from the soil, and hence there is very little 
need in the plant for water-conductive tissue. However, moss 
leaves have strands of long cells which may serve as channels of 
water movement, but such channels never reach the degree of 
specialization that they do in trees and shrubs. Mosses and 
liverworts are regarded as being more advanced than thallus 
plants. Not only have they more complex vegetative structures, 
but also the methods of reproduction are more Jn'ghly specialized. 

3. Ferns, horsetails, and club mosses (pteridophytes). This 
is a large assemblage of plants which have structures enabling 
them to live on the land, and bring their leaves up in the air, 
distant from the immediate source of water the soil. In order 
to do this there must be in the stems and leaves a certain amount 
of strengthening tissue, and also special conductive tissue for the 
rapid movement of water, mineral salts, and foods. This group 
of plants the pteridophytes includes the first vascular plants, 


that is, plants with vascular bundles. As stated, mosses and 
liverworts do not have vascular bundles. In this particular, as in 
methods of reproduction, they are more primitive than pterido- 
phytes. None of the thallophytes, bryophytes, or pteridophytes 
have seeds. 

4. Seed plants (spermatophytes). Of the four large groups of 
plants, this one has the greatest number of species, and has most 
successfully occupied the surface of the earth. The one outstand- 
ing characteristic of the group is the seed-bearing habit. The 
seed is essentially a young embryo plant, in a dormant state, sur- 
rounded by protective coats, and accompanied by a reserve of 
food. Thus, the young plant in the seed may live for years. 
Moreover, seeds often have devices, such as wings, barbs, prickles, 
etc., which facilitate their spread over the earth's surface. In 
addition to the seed-bearing habit, members of this group have 
developed extensive vascular systems, and strengthening tissues, 
enabling them to attain great heights, as witness the tall trees of 
the forests. 

Problem 3. How are the seed plants classified? 

Each of the four large groups of plants, briefly described 
above, is divided into subgroups, and these in turn into smaller 
groups, and so on. As an illustration, let us consider a seed plant, 
such as common wheat. Wheat belongs to the grand division 
of the plant kingdom known as the spermatophytes. It is a seed 
plant. Among seed plants there are two very distinct sub- 
groups, which we will call classes. There are those seed plants, 
such as pines, spruces, firs, cedars, etc., which do not bear flowers, 
as we ordinarily understand that term, and which have naked 
seeds, that is, seeds without any surrounding covering except 
the seed coats. This class is called gymnosperms. And there 
are those seed plants which have flowers, and seeds borne in a 
case, such that at some part of its life the seed or seeds have a 
surrounding covering in addition to the seed coat. This class is 
called angiosperms. Common wheat falls into this second class. 
It is an angiosperm. But the angiosperms is a very large group 
of plants. Study of large numbers of plants belonging to the 



group has shown that all of them fall naturally within two sub- 
classes. In one subclass, the so-called monocotyledons, there is 
but one seed-leaf or cotyledon in the embryo and seedling; in the 
other subclass, the so-called dicotyledons, there are two seed- 
leaves or cotyledons in the embryo and seedling. Common 
wheat is a monocotyledon. This subclass of plants, as all other 
subclasses, is subdivided into smaller groups, called orders. 
For example, wheat belongs to the order Graminales y one including 

FIG. 195. First-year and second-year carpellate cones of pine (left), and 
staminate cones (right). 

all .grasses and sedges. The plants belonging to this order have 
certain outstanding characteristics, such as a large supply of food 
stored in the seed, and inconspicuous flowers with surrounding 
scales, and no petals or sepals. All orders of plants are subdivided 
into families. An order may possess few or many families. For 
example, the order Graminales has but two families, Grarnineae 
(grasses), and Cyperaceae (sedges, rushes, etc.). Wheat is a 
grass (family Gramineae). All plant families are subdivided into 
genera (singular, genus). For example, in the grass family there 
are such common genera as Triticum (wheat), Avena (oats), 


Hordeum (barley), Zea (corn), etc. As a rule, in any genus of 
plants there are a number of different species. Common bread 
wheat is Triticum aestivum] Polish wheat is Triticum polonicum; 
durum wheat is Triticum durum] etc. So, in the system of 
classifying plants now in use, the common bread wheat is a plant 
which has characteristics such as to place it in the genus Triticum, 
which in turn is a member of the grass family (Gramineae) ; this 
belongs to the grass order (Graminales) ; this, one of the orders 
of the larger groups, belongs to the subclass, monocotyledons; this, 
in turn, belongs to one of the subdivisions of the class angiosperms, 
or flowering plants; and this class belongs to the grand division, 

The system of classifying as applied to common wheat may be 
shown in diagram form as follows : 

Grand Division Spermatophytes 
Class Angiosperms 

Subclass Monocotyledons 
Order Graminales 
Family Gramineae 
Genus Triticum 
Species aestivum 

Problem 4. What is a scientific name? 

In giving the scientific name of bread wheat, for example, we 
use both the generic and specific names: Triticum aestivum. 
Triticum aestivum is a binomial; that is, it consists of two names, 
Triticum and aestivum. Triticum, alone, refers to all kinds of 
wheat. Triticum is the genus to which all wheats belong. A 
certain kind or species of wheat, the common bread wheat, having 
certain well-defined characteristics, is called Triticum aestivum, 
aestivum being the specific name. However, aestivum alone 
means nothing; it must be joined with the name of a genus in 
order to refer definitely to a certain kind of plant. 

Every known different kind of plant in the world has been 
given a scientific name which is the same in all languages. Com- 
mon names vary from country to country. For example, whereas 


the English know the great bread cereal as wheat, the Germans 
call it Weizen, the French ble, etc. ; but the world over in scientific 
language it is Triticum aestivum. From the scientific standpoint 
the advantage of this is apparent. If one looks through any 
nursery or seed catalogue, he will note that reference is made to 
many plants, not only by their common names, but by their 
scientific names as well. 

Whenever a new species of plant is discovered somewhere in 
the world, some botanist (systematic botanist), usually a specialist 
in the group to which the plant belongs, writes a description of it, 
and gives it a name. This description and name are published 
in some one of the many scientific journals of the world, and the 
specimen from which the description was made constitutes a type 
specimen and is filed in some herbarium. It should be stated 
that, of the hundreds of thousands of different species of plants 
known to man, there exists somewhere in published form a 
description of each. The individual who describes a new species 
places after it his name or an abbreviation of his name. For 
example, when we see the scientific name of common oats written 
Avena saliva L., we know that the " L." is the abbreviation for 
Linnaeus, an early Swedish botanist, who first described common 

Suggested activity. Using a plant manual or nursery catalogue, record the 
scientific names of 20 common plants. Find out from an unabridged dictionary 
the derivation and meaning of the specific names. Are they descriptive of 
some character of the plant, or of its habitat, or of its distribution? 

Problem 5. What do we mean when we speak of " simple 
plants " and " complex plants "? 

Bacteria and blue-green algae are among the simplest of 
plants because their whole body consists merely of a single cell, 
or groups of similar cells. There are no special organs to perform 
this and that function. All the activities of the plant body are 
carried on in the single cell. Moreover, the cells themselves are 
simple in structure, in that they have no definite nucleus, no 
plastids, and but few special and definite cellular structures. 
We would regard the plant body of Spirogyra, for example, as 



more complex than that of a blue-green alga or a bacterium. In 
each Spirogyra cell there is a definite nucleus and a well-defined 
plastid or plastids, and the cells that are joined end to end to 
make up the plant body do not all behave alike, for some of 
them form reproductive bodies, whereas others do not. Thus, 
there is within the Spirogyra plant body some differentiation, 
which leads us to believe that Spirogyra is a more complex plant 
than any of the blue-green algae. In still so-called higher, that is, 
more complex, algae, there are special cells which act as hold- 
fasts, others which produce male reproductive organs, others 
which may act as protective organs. Thus there is further 
differentiation or increase 
in complexity. 

As another illustration 
of the difference in the com- 
plexity and degree of ad- 
vancement of plants, let us 
consider flowering plants. 
Now, there is no evidence 
that all the known kinds 
of flowering plants in the 
world today came into ex- 
istence at any one time. 
Quite the reverse is true. 
There wereflowering plants 
with characters that we 
regard as " primitive/' 
and geologic record lends 

evidence that such plants appeared on the surface of the 
earth earlier in time than the more " advanced " kinds. More- 
over, these " primitive " flowering plants are the ancestors of 
those which came after them. Primitive flowering plants, for 
the most part, had flowers with separate carpels with superior 
ovary, with numerous stamens, with regular symmetry, and with 
separate petals. The ordinary buttercup is a flower of this type. 
As time passed, and development among flowering plants took 
place, it is evident from a many-angled study that there were 
certain tendencies in the development of flowering plants; the 

Fia. 196. The orchid is one of the most 
advanced of the flowering plants. 


direction of development was along rather definite lines. For 
example, there was a tendency for development from separate 
carpels to united carpels; from separate petals to united petals; 
from regular flowers to irregular flowers; from numerous stamens 
to a definite number of stamens (usually less than ten) ; and from a 
superior ovary to an inferior ovary. The harebell has all the 
" advanced " characteristics just noted, and for those reasons 
would be considered a type of flower which has progressed farther 
in its development and degree of complexity than the buttercup. 
There are many other tendencies besides those mentioned which 
enable us to judge of the relative complexity or advancement of 
flowering plants. For example, wind pollination is usually asso- 
ciated with more primitive flower types than is insect pollination. 
So, as botanists have studied the great array of different flower- 
ing plants which populate the earth, they have attempted, after 
thorough study of all their characteristics, to place them in groups 
and subgroups which show their actual affinities, or relationships, 
or degree of advancement. In other words, botanists have 
attempted the construction of natural systems of classification. 
For example, the arrangement of different flowering plants into 
certain classes, orders, families, genera, and species is by no means 
an arbitrary one, made for man's convenience, but the particular 
arrangement adopted is one which conforms to the natural rela- 
tionships of the plants considered. 

Suggested activities. 1. Grow different kinds of algae in the laboratory or 
at home. Much can be learned of the nature of these simple plants by setting 
up suitable conditions and growing the plants indoors. Collect pieces of 
bark showing a green coating of Protococcus from the north side of trees in 
the woods. Place the pieces of bark, green side up, in a soup dish, moisten, 
and cover with a pane of glass. The light conditions of a north window are 
suitable. Moderate temperature and light conditions and moist air are neces- 
sary for the growth of the Protococcus on the pieces of bark. Scrape a small 
amount of the green material and mount in water under a cover-glass. Exam- 
ine with the low and high power of the microscope. What characteristics place 
Protococcus in the thallophyte group? 

Collect green masses of plant material found floating free or attached to 
sticks and stones in streams or ponds. Arrange jars in moderate light in 
the laboratory, and place in separate jars a small quantity of each specimen of 
alga with water from the pond or stream in which it was found growing. Cover 
each jar with a pane of glass and let it stand for observation. What character- 


istics do you observe without or with the aid of the microscope which place 
these algae in the thallophyte phylum? 

2. Make a collection of dry fungi and arrange as a laboratory demonstration. 

3. Grow mosses in the laboratory or at home. Make a collection of various 
kinds of mosses, and arrange growing conditions for them as follows: Place in a 
glass aquarium 2 inches of rich woods soil and over this arrange the different 
mosses which you have collected. Moisten the soil and mosses well and cover 
with a pane of glass. A north window affords suitable light for most mosses. 
Note the characteristics which place mosses in the Bryophyta. 

4. Make a collection of small fern plants and establish a fernery in the 
manner described above for the mosses. 

5. Prepare a laboratory or class demonstration of an entire fern plant 
showing spore-bearing and other fronds, underground stem, and roots. Why 
does the fern belong to the pteridophyte group? 

6. Make a collection of seeds of dicotyledons, as bean, pumpkin, and 

7. Make a collection of seeds of grasses (monocotyledons), as corn, oats, 
and wheat. 


Flower Families and Ancestors, by F. E. and E. S. CLEMENTS, published 
by H. W. Wilson Company, New York, 1928. 156 pages, 64 illustrations. 
This includes a full-page colored flower chart. "The present book has been 
written in the hope of making the study of flowering plants both simple and 
attractive to beginners of all ages." Among the interesting topics are the 
following: The family tree, the work of flowers, standardized methods in 
pollination, efficiency in flowers, evolution and relationship of flowers. 

Flowers and Flowering Plants, by RAYMOND J. POOL, published by 
McGraw-Hill Book Company, New York, 1929. 378 pages, 191 illustrations. 
An introduction to the nature and work of flowers and the classification of 
flowering plants. 


We have learned that all animal life on the earth, including 
man, is dependent upon green plants. Green plants are the only 
organisms on this earth which possess the power of converting 
the energy of light into food. All non-green plants and all animals 
derive their food directly or indirectly from green plants. Thus, 

FIG. 197. Trees and other plants help to make this country place a real farm 


the very life of man on the earth depends upon the activity of green 

The great civilizations of the world have developed where 
natural conditions favored the cultivation of certain food plants, 
chiefly cereals. Rice, wheat, corn these have been the three 
most important food plants which have made possible the develop- 
ment of three great civilizations : (1) that which spread over eastern 
Asia, Japan, the Indian Archipelago, the Malay Peninsula, and 



the Philippine Islands was dominated by rice; (2) that which 
developed in western Asia, northern Africa, and Europe had wheat 
and related cereals as its chief food plants; and (3) the physical, 
social and religious life of the Mayas, Aztecs, Incas, Guatemalans, 
Peruvians, and other aboriginal American peoples was based on 
maize or Indian corn. 

The plants of economic importance to man fall into two large 
groups, namely, (1) those that are useful, and (2) those that are 
harmful, or interfere with man's operations. The number of 
products of plant origin is enormous ; those useful to man in one 
way or another may be grouped as follows: foods; industrial 
plants including wood, coal, cork, fiber, resins, and turpentine, 
gums, plant dyes, fixed and volatile oils, and rubber; medicinal 
plants; and ornamentals. Those plants which interfere with 
man's operations include weeds, poisonous plants, hay-fever plants, 
and those which cause plant and animal diseases. 

Problem 1. What are the principal food plants of the world? 

The principal food plants of the world include the cereals, 
fruits, nuts, vegetables, beverage plants, sugar plants, and spices. 

Cereals. The principal cereals are wheat, oats, barley, rye, rice, 
corn (maize), sorghums, millets, and buckwheat. All the cereals, 
with the exception of buckwheat, belong to the grass family 
(Gramineae). A cereal is a grass grown for its grain. The great 
importance of cereals is due to the fact that a large reserve of food 
is stored in the seed. Starch is the chief food reserve of such seeds. 

By far the largest proportion of the world's supply of flour is 
made from wheat. There are a number of economic types of 
wheat, chief of which are common bread wheat, durum, club, and 
emmer. Durum wheats are used extensively in the manufacture 
of macaroni, spaghetti, and semolina. Emmer is used as a feed 
for livestock, and to some extent in the manufacture of breakfast 
food. Common bread wheat and club wheat are the ones ordi- 
narily used for flour. Oats are consumed in large amounts in the 
form of rolled oats or oatmeal. It is also valued as horse feed. 
Barley has a great variety of uses: preparation of malt, flour, 
cereal breakfast foods, stock feed, and hay. Rye flour is made into 


FIG. 198. The six principal types of corn. From left to right, pod corn, soft 

corn, flint corn, dent corn, sweet corn, and pop corn. (After Montgomery, from 

Robbing, in Botany of Crop Plants.) 

bread; the grain is fed to stock; the straw finds considerable use 
in the manufacture of paper strawboard, hats, and other coarse 

FIG. 199. Grain of corn. A, median lengthwise section, cut parallel to broad 

surface, of grain of dent corn; B, cross-section of the same through the embryo; 

C, section as in A of flint corn. (From Robbins, in Botany of Crop Plants.) 



straw articles. No other cereal is put to such a variety of uses as 
is corn. The grain and fodder are both valued stock feed; in 
addition, there are such products as corn meal, cornflakes, starch, 
glucose, etc. There are two types of sorghum, the sweet sorghums 

FIG. 200. Panicle of rice (Oryza saliva). (From Robbins, in Botany of 

Crop Plants.) 

from which a syrup is made, and the non-saccharine or grain sorg- 
hums, some of which are raised for the grain and others from which 
brooms are made, utilizing the flower stalks. Rice is a food for 
more human beings than any other grain. The millets are grown 


chiefly as a hay crop, for pasturage purposes, and for the seeds. 
The principal use of buckwheat is in the manufacture of pancake 

Suggested activities. 

1. Locate on an outline map of the United States the principal corn-growing 
regions and the principal wheat-growing regions. 

2. Prepare a paper on the manufacture of brooms, 

3. Prepare a report on the milling of rice. 


1. What are the differences between " soft wheat " and " hard wheat "? 

2. What is the " gluten " of wheat? 

3. What is " bran " of wheat? 

4. What are the differences between graham, entire wheat, and patent or 
straight bread flour? 

5. What is meant by spring wheat? Winter wheat? 

6. What is the relation between " wild oats " and ordinary cultivated oats? 

7. What is the corn silk? The corn tassel? 

8. How do you explain the occurrence of different colored grains on an ear 
of corn? 

FIG. 201. A very old grape vine, in Carpenteria, California. (Photograph 
furnished by Division of Pomology, California College of Agriculture.) 



9. What are the differences between popcorn and other types of corn? 
Account for the popping qualities of the former. 

10. What are the qualifications of a good malt barley? 

11. Name five important breakfast foods and the cereals from which they 
are made. 

Fruits. In a popular sense a " fruit " is a juicy structure eaten 
chiefly for its sweet or acid juice. This is the sense in which it is 
used here. Strictly speaking, a fruit is the matured ovary of a 

FIG. 202. Northern fox grape, Vitis labrusca; leafy flowering stem. (From 
Bobbins, in Botany of Crop Plants.) 



flower and, in some instances, other flower parts. Used in this 
sense, it would include grains, nuts, and such common " vege- 
tables " as peas, beans, squash, etc. 

In temperate climates the more common fruits are found in 
the following plant families: palm family (date); mulberry 
family (mulberry, fig); gooseberry family (gooseberry, currant); 
rose family (raspberry, blackberry, dewberry, strawberry) ; apple 

FIG. 203. Meserve Avocado growu at Long Beach, California. (Photograph, 
courtesy of Professor Ira J. Condit, University of California Branch of the 
College of Agriculture at Los Angeles. (From Robbins and Ramaley, in Plants 

Useful to Man.) 

family (apple, pear, quince) ; plum family (plum, cherry, apricot, 
peach, nectarine); citrus family (kumquat, orange, lemon, grape- 
fruit, lime); grape family (grape); potato family (tomato); 
cucurbit family (watermelon, muskmelon); olive family (olive). 

The tropics produce a great many edible fruits. Chief of 
them are the banana, pineapple, mango, avocado, and papaya. 



Others of less importance, at least to us in temperate climates, 
are the cherimoya, sugar apple, soursop, loquat, guava, Japanese 
persimmon, and mangosteen. 


1. In southwestern United States, where the date palm is grown, it is usu- 
ally propagated by the offshoots rather than the seeds. Explain why this is 
the practice. 

2. What is the relation of the mulberry tree to the silk industry? 

FIG. 204. Date palms, Phoenix dactylifera. Garden of Deglet Noor dates in 
full bearing, southern California. (After Nixon, from Robbins, in Botany of 

Crop Plants.) 

3. In order to grow Smyrna figs it is necessary to introduce into the or- 
chard the fig wasp. Explain. 

4. What are the differences between currants and gooseberries? 

5. How does the blackberry fruit differ from that of the raspberry? 

6. What is the loganberry? 

7. How is vinegar made from apples? 

8. What is a prune? 

9. What is the relation of the nectarine to the peach? 

10. What is the difference between a peach and a nectarine? 

11. What is commercial " citron "? 


Nuts, The nut is a fruit, botanically speaking. It is eaten 
for the edible kernel which is usually protected by a hard shell. 
Nuts are rich in protein and oil. The principal nut-bearing 
families are as follows: walnut family (walnut, butternut, pecan 
and hickory nut); birch family (filbert or hazelnut); beech or 
oak family (chestnut); plum family (almond); pea family 

FIG. 205. Pineapple, Ananas sativus (Bromeliaceae), growing in a Florida 
garden. (From Gager's General Botany.) 

(peanut); palm family (coconut); myrtle family (Brazil nut); 
and cashew family (pistachio). 

Vegetables. In a popular sense a " vegetable " may either be 
a vegetative structure of the plant, such as roots, stems, or leaves, 
or a reproductive structure, that is a true fruit, botanically. For 
example, the part of the lettuce plant used for food is the leaves, 
and of the potato plant, the tubers. In both these vegetables, 


vegetative parts of the plant are used. On the other hand, the 
squash, commonly called a " vegetable/ 1 is in reality a true fruit, 
being derived from the flower, a reproductive structure. 

The common families producing " vegetables " which are dug 
from the soil are as follows: potato family (Irish potato) ; morning- 
glory family (sweet potato) ; lily family (onion, garlic, leek, chive) ; 
goosefoot family (garden beet); carrot family (carrot, parsnip); 
mustard family (turnip, rutabaga); composite family (Jerusalem 

FIG. 206. Peanut, Arachis hypogaea] entire plant, reduced. The flower stalks 

after pollination grow downward and the fruit is ripened underground. (After 

Jones, from Robbins, in Botany of Crop Plants.) 

The leafy vegetables include those known as salad plants or 
pot herbs. Chief of these are: lily family (asparagus); mustard 
family (cabbage, kohlrabi, kale, borecole, Brussels sprouts, water- 
cress); goosefoot family (spinach); carrot family (celery, parsley); 
composite family (lettuce). 

The so-called fruit vegetables include representatives of the 
following: cucurbit family (squash, pumpkin); potato family 
(tomato, eggplant). 



1. Why are potatoes grown from the true seeds not true to type? 

2. The Burbank variety of potato was developed from the true seed. 
How is the variety kept true to type? 

3. What is the native home of the potato? 

4. What is the principal food stored in the potato? 

5. How do ordinary sweet potatoes differ from " yams "? 

6. What gives the red color to the common garden beet? 

7. Find out what you can about the manufacture of sugar from the 
Jerusalem artichoke. 

Beverage plants. Each of the three ancient centers of agri- 
culture has furnished to the world a valuable non-alcoholic 
beverage. Tea (Thea sinensis) is a native of the orient; coffee 
(Coffea arabica) came originally from the Mediterranean region; 
and the cacoa tree (Theobroma cacao), the source of chocolate, 
belongs to the American tropics. Commercial tea is the leaves of 
the plant, the coffee of commerce is the seeds, and commercial 
chocolate is also the seeds. What is the difference between 
" green tea " and " black tea "? What are the chief coffee- 
producing countries? 

Sugar plants. The world's supply of sugar is derived chiefly 
from two plants, sugar cane, and sugar beet. Sugar cane 
(Saccharum officinarum) is a member of the grass family, and a 
native of the tropics. The sugar beet (Beta vulgaris) is a member 
of the goosefoot family, and is grown in temperate climates. The 
juice of sugar cane is derived from the stalks, that of the sugar 
beet from the roots. The sugar extracted from the juice of these 
two plants is identical chemically. It is sucrose or cane sugar 
(Ci2H220n). Do you think there is any difference between 
sugar from the beet and that from the cane in sweetening power, 
or in its behavior when used in making candy, ice-cream, canned 
fruit, jellies, and preserves? 

The sap of the sugar maple tree also supplies a considerable 
amount of sucrose sugar. A species of palm (Phoenix sylvestris) 
has long been a source of sugar in India. What is the average 
sugar percentage of cane? Of sugar beet? What is the food 
value of sugar? What are the principal sugar-beet-producing 
countries? Where is cane grown chiefly? 



Spices and flavoring substances. A great number of plants 
yield spices. Cinnamon is derived from a number of different 
Asiatic trees of the laurel family. Black pepper is derived from 

FIG. 207. Coconut palm, Cocos nucifera, growing in central Siam. (Photo- 
graph by courtesy of Dr. Gordon Alexander. From Robbins and Ramaley, in 
Plants Useful to Man.) 

the outer part of the unripe fruit of a woody vine (Piper nigrum), 
cultivated throughout the old world tropics. The inner stony 
part of the fruit of this same plant is the source of white pepper. 


Cloves are a product of an evergreen tree, Eugenia aromatica, 
of the myrtle family. Nutmeg is the seed and mace the branched 
fibrous outer coat of the seed belonging to a tree, Myristica fra- 
grans, which grows wild in the Molucca Islands and New Guinea. 
Ginger is the rhizome of a tall herbaceous canna-like plant, 
Zinziber officinale, a native of Asia south of the Himalayas. 
Cayenne pepper is derived from the tropical American plant, 

FIG. 208. Coffee, Coffea arabica (Rubiaceae) ; young trees in the Dutch Gov- 
ernment Agricultural Department plantation, Buitenzorg, Java. (From Rob- 
bins and Ramaley, in Plants Useful to Man.) 

Capsicum. The principal flavoring substances are peppermint, 
wintergreen, lemon, and vanilla. Peppermint is extracted from 
the whole plant of a member (Mentha) of the mint family; winter- 
green from the leaves of a heath-like plant (Gaultheria)', lemon 
from the rind of the fruit; and vanilla from the vanilla bean, 
a tropical plant (Vanilla planifolia) belonging to the orchid 



Food for livestock. The land animals employed by man as 
food and as beasts of burden are herbivorous, that is, they live 
directly upon plants. The plains, prairies, mountain lands, 
pampas, and other grass lands of the world support enormous 
numbers of livestock. Man has cultivated many plants for the 

FIG. 209. Pepper, Piper nigrum (Piperaceae); climbing upon a tree in a 
tropical garden. (From Robbins and Ramaley, in Plants Useful to Man.) 

use of domesticated animals. Chief of these are the cereals, 
timothy, Sudan grass, and many other grasses, various clovers 
(Trifolium), alfalfas (Medicago), and certain root crops, such as 
mangel-wurzels, rutabagas, swede turnips, etc. 


Problem 2. What are the principal industrial plants? 

In addition to the common plants which yield food for man 
and beasts, we may consider also the following industrial plant 
products: (1) wood; (2) coal; (3) cork; (4) fibers, straws, and 
twigs; (5) resins and turpentine; (6) gums; (7) vegetable dyes; 
(8) fixed and volatile oils; and (9) rubber. 

FIG. 210. Upland cotton, a fiber plant of great economic importance, 

Fiber plants. The most important fiber plants in the world 
are cotton (Gossypium), flax (Linum), hemp (Cannabis), sisal 
(Agave species). Fibers are used for a great variety of purposes: 
fabrics of all kinds, cordage, brushes, matting, paper, filling, 
plaiting, etc. 

There are more than 40 species of Gossypium. The mature 



fruit called the " boll " is filled with seeds, which are covered with 
hairs, the cotton fibers. The cotton fiber is thus classed as a 
" surface fiber. " As a rule, there are two kinds of hairs on the 
seed: (a) the long hairs, the so-called lint or commercial fiber; 
and (6) short hairs or fuzz. The principal varieties of cotton are 
the Sea Island cottons, Egyptian cottons, and American Upland 
cottons. The finer threads 
are made from Sea Island 
cotton; ordinary threads and 
yarns are from Upland cotton. 
What is a cotton gin? 

The flax plant (Linum- 
usitatissimum) is a slender an- 
nual plant which has been used 
for its fiber as far back as the 
Swiss dwellers of the stone age. 
The fiber of flax is in the bark. 
It is known as "bast fiber." 
From the fiber is made the 
linen of commerce. Our 
finest linens are from foreign- 
grown flax, the best known 
of which are the Flemish, 
which is grown in Belgium, 
and , the Irish linen. Flax 
fiber is also employed for mak- 
ing thread, fishing lines, seine 
twines, canvas, and duck. 

Hemp (Cannabis) is a rep- 
resentative of the mulberry 
family. The stem yields a 
fiber which is the strongest and 
most durable of soft fibers with 
the single exception of flax. 
Like flax, hemp is a " bast 
fiber." Hemp fibers are used 

in the manufacture of sail cloth, yacht cordage, binder twine, 
sacking, bagging, rope, etc. 

FIG. 211. Flax. A, floral diagram c, 
calyx; co, corolla; s, stamens; p, pistil. 

B, Median lengthwise section of flower. 

C, calyx and corolla removed. D, fruit, 
external view. E, cross-section of fruit. 
(From Robbins, in Botany of Crop 


FIG. 212. Hemp. 

Fiber hung up to dry. Coconut palm in background. 
(From Gager's General Botany.) 

FIG. 213. Manila fiber plants, Musa textilis, growing in an experimental 
garden at Buitenzorg, Java. (From Robbins and Ramaley, in Plants Useful 

to Man.) 



The chief fiber competing with hemp is jute. Jute is produced 
in India from two species of plants (Corchorus species) of the 
linden family. It is used extensively for the manufacture of 
sugar sacks, gunny sacks, burlaps, grain sacks, and wool sacking. 

The husk of the coconut, a tropical tree of the palm family, 
yields an inferior fiber which is employed in making coir rope and 

Manila fiber, sometimes called " Manila hemp," is derived 
from Musa textilis, of the Philippine Islands, a plant closely 
related to the common banana. The fiber, known as a " hard 
fiber," is obtained from the flower stalk 
and leaf bases. Older and coarser fiber is 
used for cordage (Manila rope and twine) ; 
the younger and softer material is used 
for fine fabrics. 

Sisal hemp is derived from species of 
Agave, growing in tropical and subtropical 
America. The so-called century plant of 
parks and gardens is Agave americana. 

Exercise 140. Microscopic examination of 
fibers. Examine microscopically the following 
fibers, and learn to recognize the important 
differences: wool, silk, cotton, flax, hemp, and 
jute. After familiarizing yourself with the fiber 
characteristics, examine different kinds of cloth, 
identifying the kind of materials of which it is 

FIG. 214. Whip made 
from bast fibers. The 
abundance, flexibility, and 
strength of the bast fibers 
of some stems is illus- 
trated by the fact that a 
Filipino lad was able to 
make this whip using the 
fibers of the bark of a na- 
tive Philippine shrub after 
the wood and parts of the 
bark were removed. 

Wood. The uses of wood are so well 
known that they need not be described in 
detail; it will be sufficient to mention a 
few of its uses as follows : fuel, building 
materials, furniture, vehicles, musical 

instruments, cooperage, boxes, watercraft, fences, poles, posts, 
and wood pulp for paper-making. What are the structural 
differences between " soft wood " and " hard wood? " 

Coal. Botanically, coal is ancient vegetation variously modi- 
fied through decay, pressure, and heat. What are the differences 
between soft coal and hard coal? 


Cork. Cork is derived chiefly from the cork oak (Quercus 
suber), a tree of the Mediterranean region. What are the various 
uses of cork? Why is cork waterproof? 

Resins. The resins of commerce are exudations of trees and 
shrubs, chiefly of the pine family. The best known resin is the 

FIG. 215. A large bamboo, Bambusa polymorpha. Peradeniya, Ceylon. (From 
Robbins and Ramaley, in Plants Useful to Man.) 

common rosin derived from one or more species of pine in our 
southern states. Rosin has a number of applications; it is an 
ingredient of varnishes, low-grade sealing wax, and cheap soaps, 
and it is used in various electrical instruments. 

Turpentine. Turpentine is a volatile substance obtained by 
distillation of the exudate of pines; it is the substance in which the 
resin proper was dissolved as it occurred in the tree. What are 
the chief uses of turpentine? 



Vegetable gums. The vegetable gums are exudations from 
the stems of many trees and shrubs. Gum arable is derived from 
a shrub, Acacia Senegal. Gum tragacanth comes from Astragalus 
gummifer of southwestern Asia. Dextrin is produced artificially 
from starch. It is used in place of the more expensive natural gums 
to make mucilage. Gamboge is the dried juice from the bark of 
an evergreen tree, Garcinia hariburyi] it is made into a bright 
yellow paint and also has me- 
dicinal uses. 

Plant dyes. Many plants 
have bright-colored juices in the 
roots or sometimes in other parts. 
One of the chief plant dyes is 
logwood, Haematoxylin campe- 
chianum, a Brazilian plant. From 
itis prepared haematoxylin, a stain 
used by microscopists. Indigo, 
from the leaves of certain shrubby 
plants of the pea family, is of 
little commercial importance to- 
day, having been replaced by 
synthetic coal-tar products. 

Oils. There are two different 
kinds of vegetable oils : (a) fixed 
oils, which make a permanent 
stain or " spot "; and (6) volatile 
oils, which do not make a perma- 
nent stain or spot. The principal 

fixed oils, and their chief uses, are as follows: cottonseed oil 
(Gossypium), food; cacao butter (Theobroma cacao), pharmacy; 
coconut oil (Cocos meet/era), food, soap; olive oil (Olea europoea), 
food, soap; peanut oil (Arachis hypogoed), food, soap; ape oil 
(Brassica napus), lubricant, food; linseed oil (Linum usitatissi- 
mum), paint, varnish; castor oil (Ricinus communis) medicine, 
lubricant; maize oil (Zea mays), food, paint; palm oil (Elaeis 
guineensis), soap, lubricant. The most important volatile oils, 
which are employed for flavoring, as medicine, and for perfumery, 
are as follows: clove oil (Eugenia aromatica), cedar oil (Sabina 

FIG. 216. A pine tree in Alabama, 
tapped for collection of turpentine. 



virginiana), nutmeg oil (Myristica fragrans), anise oil (Pumpi- 
nella aniswri), thyme oil (Thymus vulgar is), wintergreen oil 
(Gaultheria procwnbens), peppermint oil (Mentha piperita), and 
lemon oil (Citrus limonid). 

Rubber. Rubber is obtained from the milky juice of many 
kinds of trees and shrubs but at the present time the Para rubber 
tree, Hevea brasiliensis, furnishes most of the world's supply. 
Crude rubber is prepared from the thick plant juice which exudes 

FIG. 217. India rubber tree, Ficus elastica (Moraceae), in the botanic gardens 
at Buitenzorg, Java. (From Robbing and Ramaley, in Plants Useful to Man.) 

from the bark after it is slashed. Somewhat like rubber is the 
substance gutta-percha derived from the milky juice of Palaquium 
oblongifolia, native of the East Indies and the Malay Peninsula. 
It is used chiefly to insulate and waterproof submarine and under- 
ground electric cables. A member of the composite family, known 
as " Guayula " (Parthenium argentatum) , has been cultivated to 
some extent in the arid sections of the United States as a source of 

Suggested activity. Find out what you can about the discovery of vul- 
canization, a process which revolutionized the rubber industry. 



1. Describe the method of wood formation in a tree. 

2. What is quarter-sawed wood? 

3. What is responsible for the grain of wood? 

4. Do you know wood that is heavier than water? 

5. Describe the method of bark formation in a tree. 

6. What is the chemical composition of cotton fibers? 

7. What is the source of the mucilage used on postage stamps? 

8. Has man been successful in making a synthetic rubber? 

9. What is the chemical composition of the milky juice or latex from which 
rubber is made? 

Problem 3. What are the principal medicinal plants? 

The early history of botany is closely associated with the 
development of medicine. The botanists of primitive peoples 
were also the physicians, priests, and sorcerers. In the healing of 
disease some plants or plant parts are employed as charms, some 
as fetiches, some as true medicines used for supposed physiological 
effect. From the earliest time the Chinese have been active in 
the use of " herbs " for medicinal purposes. They have collected 
roots, berries, and barks, and made from them various extracts, 
decoctions, and infusions. 

It is customary in modern medicine to judge of the value of 
drugs by their physiological action upon lower animals. The 
so-called " active principle " in a plant or plant part may be an 
alkaloid, a glucoside, a fixed or volatile oil, a resin, or some other 
specific chemical substance. 

Examination of the Official United States Pharmacopoeia, 
which is an authoritative book containing the formulas and 
methods of preparation of medicines, etc., for the use of druggists, 
will reveal the fact that many hundreds of plants yield drugs. 

However, there are now many more different kinds of drugs 
than are really necessary or desirable, for numerous drugs of plant 
origin have essentially the same physiological action. The most 
important drug plants in the world are as follows : 

1. Poppy (Papaver somniferum). The dried juice, known as 
opium, from the capsule, is a great reliever of pain. Opium is 
obtained from the unripe fruits which are cut with a knife to allow 


the milky juice to exude. This juice when dried forms the opium 
of commerce and contains about 10 per cent of the alkaloid 

2. Cinchona (Cinchona spp.), a member of the madder family. 
This medium-sized evergreen tree yields a bark known as Peruvian 
bark, which furnishes quinine, a specific cure and preventive of 
malaria. Quinine is a white amorphous or crystalline powder, 
very bitter to the taste, which exists in cinchona bark to the 
amount of 5 to 10 per cent. It is one of the very few specific 

Fia. 218. Cinchona in fruit, photographed from a herbarium specimen. 
(From Robbing and Ramaley, in Plants Useful to Man.) 

medicines. Today the Dutch have made their Javanese planta- 
tions " the throne of the quinine trade." The Dutch are prac- 
tically the only exporters of cinchona in the world. They have 
about 25,000 acres under plantation. 

3. Digitalis (Digitalis purpurea). This plant is a member of 
the figwort family. The leaves contain various glucosides, chief 
of which is digitalin, a drug which slows up heart action, and hence 
is used in the treatment of certain cases of heart disease. 



4. Belladonna (Atropa belladonna). This plant, a member of 
the nightshade family, to which also belong potato, tomato, 
tobacco, petunia, etc., yields a drug known as atropine, an alkaloid 
employed by oculists to paralyze those muscles of the eye which 
cause accommodation. 

FIG. 219. Chaulmoogra; young tree on the grounds of the leper hospital at 

Chiengmai, Siam. (Photograph by Dr. Gordon Alexander. From Robbing 

and Ramaley, in Plants Useful to Man ) 

5. Chaulmoogra tree (Taraktogenos kurzii). This plant, also 
known as " Kalaw," is a tall jungle tree of north Burma, which 
yields Chaulmoogra oil, employed in the treatment of leprosy. 

Other plant drugs of importance are as follows : gum arable, a 
mucilaginous substance which is an exudation from the trunk and 


branches of the gum arable tree (Acacia Senegal)] castor-oil, 
obtained from the seeds of the castor-oil plant (Ricinus corn- 
munis)] calamus, from the underground stem of sweet-flag 
(Acorns calamus) ; camphor, a gum-like drug obtained by distil- 
lation from the wood of the camphor tree (Cinnamomum cam- 
phora}] menthol, from the volatile oil of peppermint; strychnine, 
an alkaloid derived from the seeds of the nux vomica tree (Strych- 
nos nux-vomica). 

Problem 4. What are the principal by-products derived 
from plants? 

Innumerable by-products are derived from plants. The chief 
product of the cotton plant is the long fiber; but there are numer- 
ous important by-products. The short lint or fuzz, known as 
" linters," which is not removed in ginning, is taken from the seeds 
and made into poor-quality twine, carpets, and batting. Now, 
" linters," 85 per cent of which is cellulose, are the basis of numer- 
ous products: leather substitutes, toilet articles of all kinds, 
kodak and movie films, rayon, fine paper, and collodion. Cotton- 
seed hulls are used in the manufacture of paper and fiber board 
from which are made gear wheels, trunks, etc.; and the hulls are 
also utilized as fuel and fertilizer, and as a cattle food. Cotton- 
seed oil is one of the most valuable products of the cotton plant. 
The oil is in the embryo of the seed. This oil is now produced in 
very large quantities, this country having exported 33,673,000 
gallons in 1921. It appears on the market as " table oil," 
" sweet nut oil," and " salad oil." Some of it is utilized in the 
manufacture of soap, also of " oleomargarine " and other butter 
and lard substitutes. Guncotton is an explosive made by treating 
cotton or some other form of cellulose with nitric or sulphuric 
acid. Those kinds of guncotton soluble in alcohol or ether are 
used in the manufacture of rayon, celluloid, etc. 

The flax plant yields both fiber and oil (linseed oil). Neither 
can be regarded as a by-product. In the manufacture of linseed 
oil from the seed of flax, the residue from the crushed seeds gives 
a cake or meal which is a valued stock food. Flax seeds are used 
whole for various medical purposes. The threshed straw of the 


northwestern seed flax is employed to some extent for upholstering, 
for insulating cold-storage plants, refrigerator cars, and ice boxes. 
The principal use of hemp (Cannabis) is as a fiber plant. The 
seeds are often fed to poultry and cage birds; the leaves and 
flowers yield a drug known as Cannabis indica; the seeds give an 
oil which is used for making soft soaps and as a paint oil, and low 
grades are utilized for certain varnishes; hemp stems make a fair 
grade of paper. 

The sugar beet (Beta vulgaris) furnishes a large proportion of 
the world's sugar supply. The by-products of the industry have 
enormous value. The tops, molasses, and pulp are valued stock 
feed, and the waste water from the manufacturing process is 
boiled down, yielding fertilizer. It has been demonstrated that it 
is possible to manufacture fusel oil, alcohol, rum, and vinegar from 
the refuse beet molasses. 

No other cereal is put to such a variety of uses as is corn. 
Some economical use has been found for nearly every part of the 
plant. It is a food for man and beast. Corn oil is obtained from 
the embryo; it is used for salads, in cooking, in the manufacture 
of soap and paints, and sometimes it is vulcanized into a cheap 
grade of rubber substitutes. The manufacture of corn starch 
consumes about 50,000,000 bushels of corn annually in the United 
States. Commercial " glucose " is a thick syrup derived by the 
partial hydrolysis of starch, and is employed as the basis of many 
manufactured jellies and preserved fruits. Artificial gums, 
known as dextrin and British gums, are made from corn starch; 
they are used on envelopes and postage stamps, and also in many 
of the textile industries. The pith from the stalks is made into 
explosives and also employed as a packing material. The stalks 
as a whole have served as a source of raw cellulosic material, from 
which numerous products can be made. Corn cobs are still in 
demand for pipes. A fine grade of charcoal is manufactured from 
corn cobs. Corn cobs also have a practical value for the produc- 
tion of furfural, paper stock, organic solvents, artificial silk, etc. 
Paper is made from the stalks, and packing for mattresses from 
the husks. Corn cake, left when oil is pressed from the embryos, 
is a stock food. And corn is the most economical source of starch 
for alcohol manufacture in the United States. 


The sugar-cane plant also yields many valued by-products. 
The molasses is used for baking purposes and as a table syrup; 
poorer grades are made into rum and alcohol, and used as stock 
food. The refuse has value as a fertilizer. Sugar-cane bagasse 
was formerly used only as fuel, but now it is made into wall 

The soy bean (Soja max) is the most important legume in 
Asiatic countries. The chief product of the bean is the oil which 
is expressed from the seeds, and the plant is grown principally for 
that purpose. The plant has a number of less important uses. 
For example, after the oil is expressed from the seed, the " cake,' ' 
either unground or ground into a meal, is used as stock feed or as 
a fertilizer. The seeds of soy beans are sometimes used as a 
substitute for coffee. 

Peanut seeds yield an oil, a nearly colorless product, employed 
as a salad oil, and to a limited extent in the manufacture of soap 
and oleomargarine. Peanut butter has become a standard food. 
Peanut meal, the product left after pressing the oil from the seeds, 
is a high-grade stock feed. 

Almond seeds yield an oil used as an ingredient of flavoring 
extracts, and the seeds are a source of prussic acid. 

Cider is the juice of apples. In the transformation of cider 
to vinegar, two fermentation processes take place; alcoholic 
fermentation, and acetic acid fermentation. The characteristic 
properties of vinegar are due to acetic acid. 

The juice of quince is sometimes employed to flavor manufac- 
tured food products. 

The buckwheat plant (Fagopyrum vulgare) is grown chiefly for 
the " grain " which is made into buckwheat flour. The " mid- 
dlings " (hulls, mixed with bran) are employed as a bedding for 
stock; and, not to be disregarded, are the flowers which produce 
an excellent grade of honey. 

Agave species are grown largely for their fiber (sisals), but 
the juice of the plant is fermented to give a drink, pulque. 

In addition to its supply of fruit, the date palm furnishes 
material for building, for ropes, baskets, and numerous other 

Rice hulls are used as a stock food; rice straw as a food for 


stock, and also in the manufacture of paper, straw hats, straw 
board, etc. 

The orange gives an important by-product in the form of an 
oil, which is employed in the manufacture of perfumes, soaps, and 
flavoring extracts. Waste oranges may be used for this purpose. 
One of the chief by-products of the lemon industry is lemon oil, 
which ranks second to vanilla extract in the quantity consumed. 

The potato plant is cultivated for its tubers, which are used 
chiefly as a food for man. But it is an important source of com- 
mercial starch and of alcohol. 

In the canning of tomatoes large amounts of refuse accumulate. 
The oil expressed from the seeds is used as a soap oil, which may 
be made into a drying oil for paint ; the meal is used as a stock food. 

Carrot roots, grown mostly for a table vegetable, contain a 
yellow pigment, carotin, which is sometimes extracted and used for 
coloring butter. 

The grape plant has a number of by-products. Brandy, feed, 
fertilizers, and acetic acid are made from the pomace. Tartaric 
acid is manufactured from the steins, shells, and the " lees " 
of wine. The seeds are used as a food for stock and as a source 
of tannin and grape oil. 

Sweet potatoes are used chiefly as a human food. Flour, 
starch, glucose, and alcohol are minor products of the root. 

Problem 5. How do plants interfere with man? 

Not all plants are useful. Many are of economic importance 
because they interfere with man's farming operations, or they in- 
jure his health or that of domestic animals. The principal groups 
of harmful plants are as follows: (1) weeds, (2) fungi which cause 
animal and plant diseases, (3) plants directly poisonous to man and 
livestock, (4) hay-fever plants. 


We have come to consider as " weeds " those plants which 
tend to grow where they are not desired; plants which tend to 
resist man's efforts to subdue them; plants which will grow in 
almost any kind of soil and under all conditions; plants which pro- 


duce seeds in enormous numbers and have other rapid methods of 
propagation; plants which in themselves sometimes are truly 
beautiful, but which for us have lost their charm; plants useless 
and troublesome. 

Losses caused by weeds. The losses caused by weeds fall into 
two chief classes: (1) losses brought about by a decrease in yield 
or quality of crop, (2) losses brought about by an increase in the 
labor cost of growing the crop. 

Weeds rob cultivated plants of water. Weeds do great injury 
in using up moisture. It is said that a large weed will use a barrel 
of water during the season. The sunflower plant requires almost 
twice as much water as corn to produce the same amount of dry 
matter; the water requirement of ragweed is about three times 
that of millet. These figures show the injury that weeds do to 
our crops through their great demand upon soil moisture. In fact, 
the main benefit derived from cultivating corn and other crops 
is in the removal of weeds which compete with^ them for soil 

Weeds crowd out and shade crop plants. By shading, weeds 
may retard in crop plants the process of food-making (photo- 
synthesis). They may even prevent seedlings from getting a 
start. Moreover, certain fungus pests develop better in the 
shade than in direct light. 

Weeds harbor insects and fungus pests. Insects and fungi 
often spread from weeds to neighboring cultivated plants. Clean 
culture about roadsides, fence rows and ditch banks is strongly 
recommended to prevent the spread of such pests. Insects 
deposit their eggs upon the weeds, and when the larvae hatch 
they migrate into the fields. Insects often go into hibernation 
somewhere near their native food plants, many of which are weeds, 
and from them they scatter to adjoining fields of cultivated plants. 
Insect pests gradually become more and more numerous until, 
native plants being insufficient for their food supply, they move 
to adjacent fields of cultivated plants. For example, the beet 
webworm prefers lamb's quarters, Russian thistle, and Atriplex 
rather than the sugar beet as plants upon which to deposit their 
eggs. Fields infested with or bordered by these weeds attract 
the webworm moths, and when these plants are exhausted by the 


larvae, they move to nearby beets. Grasshoppers are always 
worse next to ditchbanks and roadsides, fence rows, and other 
waste land, overgrown with weeds and grass. Grasshoppers rarely 
lay their eggs in cultivated fields, but select the native haunts in 
preference. Potato bugs flourish on greenberries, nightshade, and 
buffalo bur. False chinchbug, which does great injury to seed 
crops, breeds and feeds during the early part of the season on 
shepherd's purse and other wild mustards. 

It is now well established that the fungus causing stem rust of 
wheat is harbored by certain grasses such as wild barley and that 
it may spread from wild barley and other grasses to wheat. Cer- 
tain wild mustards may serve as a host for the fungus causing 
" club-root " of cabbage. 

Weeds retard the work of harvesting grain. Weeds increase 
the pull for the horses and cause an extra wear and tear on machin- 
ery. Dodder may so mat alfalfa plants together as to make 
harvesting extremely difficult. Weeds increase the labor of 
threshing, and make an added cost in cleaning the seed. 

Weeds and dockage. One of the most serious losses occa- 
sioned by weeds in fields results from the infestation of the grain or 
seed crop. Farmers annually haul thousands of tons of weed 
seeds, chaff, and other inert matter to the mill in their wheat. 
And, of course, they are docked for this unclean wheat, and rightly 
so. Moreover, unclean seed means that the fields which produced 
the seed were weedy. And, what is worse, it means that the next 
crop grown from such seed will be weedy. 

Some weeds injure stock. The beards of downy brome grass, 
wild barley, and certain other grasses may work into the gums of 
animals, causing ulcers and the loss of teeth. Some weeds, such 
as cocklebur and sandbur, injure wool and disfigure the tails and 
manes of horses. A number of weeds are poisonous to stock. 
Not only do weeds decrease the crop yield, but when they are 
eaten they may also cause the death of stock. 

Weeds retard the drying of grain and hay. Many weeds are 
succulent and hold moisture, thus retarding the drying of crop 
plants with which they are mixed. 

Some weed seeds, such as cockle, damage the quality of 
dairy products. Weeds such as common ragweed, wild onion, 


and wild garlic, when eaten by cows, impart disagreeable flavors 
to milk, butter, and cheese. 

Why weeds are successful plants. Seed production of weeds. 
Many weeds produce an enormous number of seeds. A large 
purslane plant will produce as many as 1,250,000 seeds; a single 
Russian thistle plant will ripen 100,000 to 200,000 seeds; tumbling 
mustard as many as 1,500,000 seeds. 

The seeds of many weeds are very small and escape notice. 
A pound of clover dodder has 1,841,360 seeds; common plantain, 
1,841,360 seeds; lamb's quarters, 604,786 seeds; Russian thistle, 
266,817 seeds; wild mustard, 215,995 seeds; wild oats, 25,943 
seeds. If 60 pounds of wheat are planted to the acre, and this 
wheat has 2 per cent of wild mustard seed, there will be dis- 
tributed over that acre 388,791 mustard seeds, or 9 seeds in 
every square foot. 

Vital weed seeds at different depths in the soil. Not only do 
weeds produce seeds in tremendous numbers, but also many weeds 
produce seeds with an ability to live a long time. The seeds of 
some weeds, when buried in the soil, may retain their power of 
germination for 15 to 30 years. This is true of the seeds of tall 
pigweed, black mustard, shepherd's purse, dock, yellow foxtail, 
chickweed, and others. 

Some weeds seeds exhibit dormancy. Not all the seeds of 
a given crop of seed may germinate the first year; some may 
remain alive in the ground for a time. This has been given popu- 
lar expression in the following statement: " One year of seed gives 
seven years of weeds." 

Weeds as a class are hardy. Weeds as a class are resistant 
to insect and fungus pests. They also have the ability to with- 
stand shading, excessive drought, temperature extremes, and other 
unfavorable conditions. Of all weedy plants, the worst are those 
with underground stems or rootstocks, which live over from year to 
year in the soil, and enable the plant to spread rapidly in all 
directions underground. These underground stems store food, 
and, although the plant is cut off above ground, new stems are 
sent up directly from below. Weeds with rootstocks are particu- 
larly difficult to eradicate. Well-known examples of such weeds 
are quack grass, poverty weed, and Canada thistle. 


Weeds spread rapidly. It has been the history of nearly all 
agricultural communities that weeds increase in abundance and 
variety, unless concerted action is taken to combat them. Almost 
every year sees the first appearance of some weed in a community, 
and usually in a few years it is prevalent. In a few decades the 
Russian thistle has spread throughout the agricultural sections of 
the West, and in some localities is now a menace. In fact, some 
sections are being abandoned on account of the Russian thistle. 
Russian thistle seeds are now a common impurity of crop seeds. 
The entire plant may break off at the ground line, become a 
" tumble weed," and be blown for miles across the open country, 
distributing its seeds as it tumbles along. 

Weeds have excellent means of seed dispersal. Some seeds, 
like those of the thistle, milkweed, sow-thistle, wild lettuce, and 
dandelion, have cottony or feather-like attachments which enable 
them to take long aerial journeys. Most seeds will float on water 
and, consequently, are carried by streams and irrigation waters. 
A number of seeds are provided with hooked prickles or barbs 
by which they attach themselves to the clothing of man or the 
hair of animals, and are thus carried from place to place. 

Impure commercial seeds. Probably no other means of intro- 
ducing weeds is so effective as the sale and distribution of impure 
commercial seeds. 

Seeds are carried in screenings, baled hay, the packing about 
trees, and in feedstuffs. Some seeds are uninjured in passing 
through the digestive tracts of animals and consequently are 
spread on the field in manure. The use of feeding stuffs containing 
live seeds may result in the spread of noxious weeds. A threshing 
machine may carry weed seed from farm to farm. Some weeds 
are dragged by plows, cultivators, and harrows from one part of 
the field to another and even to adjacent farms. This is true of 
those perennial weeds with underground stems which are cut up 
into pieces by cultivating implements. 

Wind and water are important agents in weed dissemination, 
Wind carries seeds, and in some instances whole plants, long 
distances. In the irrgated sections, water is one of the chief means 
of spreading seeds. Ditch banks are densely overgrown with 
weeds, which shed their seeds in the water; the seeds are carried 


down stream, given a good soaking in transit, and planted 
on a well-soaked soil all conditions being ideal for germi- 

Birds may be responsible for the distribution of weed seeds. 
However, birds probably do more good in eating weed seeds than 
harm in distributing them. 

Underground spreading of weeds. Perennial weeds, such as 
Canada thistle, travel considerable distances each year under- 
ground. A small patch in one corner of a field may appear harm- 
less enough, but it may soon spread over a whole field by means 
of underground growth alone. 

Classes of weeds. Weeds fall into three classes according to 
their length of life. It is necessary to know to which class a weed 
belongs before one can wisely proceed to eradicate it. These 
weed classes are : 

1. Annuals. Those that live one year, such as Russian thistle, 
pigweed, wild oats, shepherd's purse, pepper-grass, foxtail, and 

2. Biennials. Those that live two years, producing seed at 
the end of the second year, such as wild carrot, wild parsnip, 
mullein, and bull-thistle. 

3. Perennials. Those that live from year to year by means of 
underground parts. They are our worst weeds, and when once 
established are difficult to eradicate. Some common perennial 
weeds are wild morning-glory, or bindweed, poverty weed, Ber- 
muda grass, dandelion, sow-thistle, and Canada thistle. 

Annual and biennial weeds. Annual and biennial weeds pro- 
duce seed but once and then die down entirely, root and all. They 
propagate themselves by seed alone. Consequently, all methods 
of controlling weeds of these two classes have for their object the 
prevention of seeding. Clearly, if they are kept from seeding, and 
pains are taken to prevent seeds from being introduced to the land 
in the many ways that are possible, annuals and biennials are kept 
in check on the farm. 

Annuals and biennials are easily killed by cultivation. The 
seeds of some weeds of these classes retain their vitality in the soil 
for several years, consequently several years of cultivation may be 
necessary. The principle of eradication of annuals and biennials 



is to prevent seeding, and to cause the seeds that are shed to ger- 
minate and then destroy the seedlings before they mature. 

There are two kinds of annuals : summer annuals and winter 
annuals. Summer annuals germinate their seeds in the spring, 
produce a crop of seed in the late summer or fall, and die down. 
The seedlings are not capable of living through the winter season. 
Russian thistle, foxtail grass, frenchweed or fanweed, barnyard 

FIG. 220. Canada thistle, a perennial weed that propagates both by seeds 

and underground stems. The illustration shows three different ages of shoots 

that have arisen from the underground stem and reached the surface, and 

several others which have not yet reached the surface. 

grass, witch-grass, pigweed, and lamb's quarters are common sum- 
mer annuals. Winter annuals come up from seed in the fall anc 
live over the winter in the seedling stage, producing flowers and 
fruit the following spring or early summer. Shepherd's purse, 
pepper-grass, and prickly lettuce belong to this class. The 
seeds of winter annuals germinate after fall rains, and the young 
seedlings are easily killed at this time by cultivation. Winter 
annuals frequent stubble fields. 


Perennial weeds. As has been stated, perennial weeds live 
from year to year without renewal from seed. They propagate 
themselves by means of both seed and vegetative growth (roots and 
underground stems). It is not sufficient to prevent seeding alone, 
although this should be done. All methods of holding in check or 
eradicating perennial weeds have for their object the starving or 
smothering of the underground growth. It is well known that, if 
the top growth of a perennial weed is cut off, new shoots are sent 
up from the stems or roots beneath ground. This means, then, 
that the underground parts possess reserve food material which is 
called upon to produce new shoots. As soon as leaves are pro- 
duced on the new shoots, the manufacture of food is begun by 
them, and some of this food, made in leaves, finds its way back 
into the roots or rootstocks which thereby gain in strength. 
A rootstock or perennial root will increase in size each year just 
as does a perennial stem above the ground; and the larger it gets, 
the more difficult it is to starve out. Old, well-established peren- 
nial weeds are very difficult to eradicate, for the reason that they 
have a large store of reserve food to draw upon in the production 
of one crop of leafy shoots after another. It is a common experi- 
ence that one may, by a thorough cultivation, kill all the top 
growth of a perennial weed, and find that the plant soon comes 
up thicker than before. A second thorough cultivation may be 
followed by like results. Each time that top growth was removed 
and new shoots sent forth, reserve food in the roots and rootstocks 
was used up. What is needed in the eradication of a well-estab- 
lished perennial weed is persistence and patience, cultivation fre- 
quent enough to keep down all leafy shoots, and no stopping 
until the object is attained. This may mean a season or two 
of bare fallow, followed by 'a crop which will permit of clean 
cultivation. Some farmers place in a cultivated crop like corn, 
potatoes, beets, or beans but do not cultivate often enough 
or thoroughly enough to prevent perennial weeds from making 

Small patches of perennial weeds can be eradicated by spading 
up the soil, raking out and burning the underground parts, and 
hoeing off every sprout as soon as it appears, during at least two 


Methods of weed control. Use of clean seed. The first 
principle in weed control is the use of clean seed. A lot of "clean 
seed" is one which contains only the larger and plumper seeds 
of the crop desired, and is free from sticks, stones, gravel, dirt, 
chaff, weed seeds, the seeds of other crops, smut balls, and small, 
shrunken seeds of the crop desired. 

Preventing weeds from seeding. The second principle in 
weed control is the prevention of seeding. This means the use 
of a hoed crop or meadow crop. Weeds in small grain cannot be 
prevented from seeding; as a result, continuous cropping to grain 
leads to foul fields. It is the inevitable result of a one-crop system. 

Cutting the weeds along roadsides and fences. Throughout 
our entire country, cultivated fields are surrounded or bordered 
by areas infested with weeds. Roadsides are the chief sources of 
weed-seed supply. It is true that states, counties, and cities have 
laws which require the mowing of weeds along roadways prior to 
their seeding. But in very few instances is the regulation ade- 
quately enforced. As a matter of fact, weeds come to maturity 
and stand man-high along our roadsides, contaminating the adja- 
cent cultivated fields and gardens. 

Giving special attention to manure and screenings. The seeds 
of many weeds are not injured by passage through the digestive 
tract of an animal. Consequently, weedy hay, weedy bedding, 
grain-carrying weed seeds, or unground screening are a source 
of contamination. Many stock foods contain unground screenings 
in which may be found many small weed seeds, such as the plan- 
tain. Cases are known in which the weed seeds from this source, 
wild oats, for example, have been carried to the field in the manure. 
Well-composted manure contains no visible seeds. All screenings 
containing weed seeds should be thoroughly ground or steamed 
before they are fed. It should be added here that weed seeds 
are destroyed by the fermentation processes which ensilage 

Use of cultivated or cleaning crops. Crops such as beets, 
potatoes, beans, and corn which permit frequent cultivation are 
rightly called cleaning crops. Practically all annual and biennial 
weeds readily succumb to cultivation, and perennials are effectively 
held in check. Continuous cropping, particlarly to small grain, 


which does not allow cultivation, inevitably leads to weedy fields. 
Cultivated crops are a necessity in any scheme to eradicate 

Rotation of crops. " Crop rotation " is practically a synonym 
of " good farming." In fact, the control of weeds is one of the 
principal reasons for crop rotating. There are farms on which 
weeds are of little consideration, either in increasing the labor 
expended or in decreasing crop yields. And these farms are ones 
on which a definite plan of crop rotation is systematically adhered 
to. One cannot expect to follow a continuous system of cropping 
without trouble with weeds. Even old stands of alfalfa frequently 
become weedy, and must be plowed up and placed in a cultivated 
or cleaning crop. 

Exercise 141. Field trip. Visit a weed-infested vacant lot or field. 
Find out by asking other people, or by the use of weed manuals, the common 
names of the different weeds which you do not already know. Is the habitat 
mesophytic, xerophytic, or hydrophytic? What weeds seem to be best fitted 
to the surroundings? Which of the weed characters mentioned in the text 
are possessed by the most successful of these plants? What plants of the 
habitat seem to be losing in the struggle for possession? Name qualities from 
the list in the text which the losing plants seem to lack. 

Exercise 142. Field trip. If you can find a weed-infested garden or field, 
compare the condition of the crop plants which are struggling against the 
effects of the weeds with the condition of crop plants in a well-kept garden or 
field. List the reasons for the difference in the condition of the crops in the 
two situations. When left to themselves, why do weeds win over crop plants 
in competition for possession? 

Exercise 143. Weeds which propagate vegetatively. Make a list of weeds 
which reproduce readily by vegetative means, and classify on the basis of the 
part of the plant which is important in vegetative reproduction, as quack 
grass, by rhizomes; purslane, by fragments of the stem. 

Suggested activity. Make a collection of fruits and seeds of weeds, classi- 
fying on the basis of means of dispersal, that is, by propulsion, by wind, and 
by animals. 

Exercise 144. Laboratory. Make a study of at least ten common weeds 
and record your data either in a table or as follows: 

1. Common name. 

2. Habitat: 

a. Where found growing. 

b. Mesophyte xerophyte hydrophyte . 


3. Habit: 

a. Mesophyte xerophyte hydrophyte 

b. Stem: height erect branching 

climbing prostrate rhizome 

succulent woody 

c. Leaves: large small medium 

simple compound succulent 

rosette mosaic 

d. Roots: primary lateral fibrous 

deep surface fleshy 

4. Reproduction: 

a. Pollination: by wind by insects by 

gravity .' 

b. Number and size of seeds 

c. Methods of fruit and seed dispersal 

d. Vegetative reproduction 

5. Length of life: annual biennial perennial 

6. Protection against animals: inedible spines offensive 


7. Native or introduced 

8. Means of control 


One may express surprise that plants, as well as animals, 
become diseased, that they get " sick," and that they may need 
care and treatment to prevent them from succumbing to various 
maladies. But such is the case. The " plant doctor," or rather 
the plant pathologist, studies the characteristic symptoms of plant 
diseases, he determines the causes and searches for preventive 
measures and remedies. The science of plant pathology has 
made very rapid strides in recent years, and the plant pathologist 
is a very necessary person in our agricultural development. 
Scores of trained plant pathologists are now connected with the 
United States Department of Agriculture and with the various 
agricultural experiment stations and colleges. 

It is estimated that the annual loss in the United States from 
potato blight amounts to $36,000,000; from wheat bunt, $11,000,- 
000; and from oat smut, $6,500,000. Cereal rust caused a loss 
of about 200,000,000 bushels of wheat alone in the United States 
in 1916. 


The causes of plant diseases. The different plant diseases may 
be classified as follows, according to their causes: 

1. Those caused by the activity of living organisms. 

a. Caused by animals, such as worms and insects. 

b. Caused by plants. 

1. Parasitic bacteria, and other fungi, and slime 


2. Parasitic seed plants. 

2. Those due to adverse non-living environmental factors such 
as the chemical and physical condition of the soil, light, heat, 
precipitation, wind, lightning, smoke, soot, gases and smelter 
fumes, which may result in nutritive disturbances in the plant. 

Diseases due to attacks of bacteria. Some of the chief bac- 
terial diseases of cultivated plants are bacterial blight of alfalfa, 
blight of apple and pear, bacteriosis of bean, crown gall of a num- 
ber of different plants, black rot of cabbage, and wilt of cucurbits. 
Bacterial diseases are often very destructive and spread rapidly. 
The bacteria gain entrance to the plant through wounds or through 
the pores (stomata) in the leaves. In the tissues of the host the 
bacteria find a suitable food supply, and there grow and reproduce 
rapidly. Bacteria are dependent organisms and draw their food 
from the host, causing in it nutritional disturbances and struc- 
tural modifications which have characteristic symptoms. 

Diseases caused by parasitic fungi. The large majority of 
plant diseases caused by plants are due to the activity of parasitic 
fungi. Well-known diseases brought on by these organisms are 
the mildews, the spots, rots, scabs, wilts, smuts, rusts, and cankers. 

Diseases caused by parasitic seed plants. There is one very 
harmful seed plant which lives a parasitic life on a number of dif- 
ferent plants, but is most harmful to alfalfa and clovers. This 
plant is dodder or love- vine. Dodder plants have slender, thread- 
like stems of a yellowish or orange color which twine and coil 
about the alfalfa plants. The dodder seeds germinate in the 
soil about the same time as alfalfa seeds. Later, the plants 
lose all connection with the soil. As the alfalfa plants grow, 
dodder keeps pace, spreading and branching extensively. Soon 
the dodder in a field may be detected by the dense growth of 


yellow, tangled stems, or by the presence of patches of stunted 
alfalfa plants, and in severe cases, by a mat of dodder and alfalfa. 
The dodder plant sends small absorbing organs into the tissue 
of the host, and takes foods from it. 

Diseases due to adverse environmental conditions. It is well 
known that hail will injure plants. Too much " alkali " in the 
soil may also cause serious injury to plants. Plants vary consid- 
erably in their ability to withstand alkali. The sugar beet, for 
example, can withstand much more alkali than corn or wheat or 
potatoes. Blueberries flourish in an acid soil, but become sick 
and die under other conditions. In certain parts of the irrigated 
sections of the West, nitrates have accumulated in the soil in 
such large quantities as to injure the vegetation and make it impos- 
sible to grow crops profitably upon them. This injury to plants 
is called " niter " injury. The accumulation of niter in the soil 
gives it a chocolate-brown color, and the accumulation is the 
result of soil conditions which favor the very great activity of 
certain nitrogen-fixing bacteria. 

Too much water or too little water in the soil will cause the 
plants to be sickly and to be seriously reduced in their growth. 
" Tip burn " of lettuce is thought to be due to a fluctuation in the 
temperature and moisture supply, particularly in the presence of 
readily available potassium and nitrogen. Intense light may 
cause sunscorch, " bronzing/' or sunscald. On the other hand, 
dense shade may cause plants to become weakly; for example, 
lawn grass in deep shade may languish and die. The injury of 
potato and cotton plants by lightning has been reported. In the 
neighborhood of cities where much smoke and fumes from manu- 
facturing establishments are present, plants are injured; sulphur 
dioxide and other gases act as toxins. It is known that shade 
trees are harmed by small quantities of illuminating gases which 
may escape into the soil from leaking pipes. 

Some plant diseases which can not be ascribed to any of the 
above causes are usually considered to be due to disturbances in 
plant nutrition. The causes of such diseases are often hard to 
determine. Any marked deficiency in one or more of the chemical 
elements essential to growth will result in an unhealthy develop- 
ment of the plant. The " pale " color of plants, a disease known 


as chlorosis, may be due to a deficiency or unavailability of either 
magnesium or iron, both of which are essential to the formation 
of chlorophyll. " Die-back " of lemons has been ascribed to the 
poor nitrifying power of the soil. 

Diseases caused by insects. So many of our crop plants are 
infected with insects that we are almost forced to the conclusion 
that there is not one without its insect enemy. In dealing with 
insect pests, two main types are recognized: that which includes 
insects with biting mouth-parts which feed on plant tissues, and 
that which includes insects with piercing and sucking mouth-parts 
which pierce the plant tissues and suck the plant juices. These 
small animals live upon the juices of the plant and reduce its 
vitality by destroying plant structures, or they feed upon the plant 
tissues, or steal the nourishment which is needed to make the 
plant grow well. Some of the principal insect enemies of crops are 
grasshoppers, cutworms, chinch bugs, alfalfa weevil, plant lice, 
webworms, woolly aphis, codling moth, scales, mites, borers, and 
leaf rollers. 

The principles of disease control. In the preceding pages we 
have discussed the various causes of diseases in plants. Let us 
briefly outline the principles of disease control. 

Determination of the cause. Of course, the first step in the 
control of a particular disease is to determine its exact cause. 
This may be difficult, at times, but a line of successful action can 
not be followed unless the cause is first ascertained. 

Knowledge of the life history. If the diseased condition is 
due to an organism, it is essential to know the life history of this 
organism, so that it can be attacked at its most vulnerable period. 
For example, in those smuts, such as bunt of wheat, which infect the 
host only in the seedling stage, a knowledge of the mode of infec- 
tion has led to a method of control which involves the destruction 
of spores on the seed and in the soil about the seed. Again, a 
knowledge of the life history of black stem rust of wheat has led 
to the eradication of the common barberry. 

Cultural methods. It is becoming well recognized that many 
plants succumb to diseases because of a weakened condition 
brought about by poor cultural practices. A poorly nourished 
and weak plant, like a poorly nourished and weak animal, is often 


more subject to the attacks of fungi than are strong vigorous 
plants. Hence, plants that are well cared for, that have ample 
water and mineral nutrients, and favorable soil, light, and tempera- 
ture conditions, such that growth is uninterrupted, have greater 
powers of throwing off diseases than plants which suffer from a 
lack of these factors. However, very vigorous plants may be 
attacked by diseases. 

Crop rotation. Our different crop plants have fungus diseases 
peculiar to them. For example, corn smut is known only on one 
common host, and that is maize. The smut spores survive in the 
soil and may infect the succeeding crop. By growing corn con- 
tinuously on a given area, spores accumulate and increase the 
chance for infection. However, if the area is planted to a crop 
other than corn, the spores lose their vitality or germinate after a 
time. Then corn may be planted again in the area without fear 
of infection from the soil. 

Consider another example, potato scab. It has been demon- 
strated experimentally that the fungus may persist in the soil for 
several years. Continuous cropping to potatoes only increases the 
soil infestation. But, if a crop not subject to the attacks of that 
particular organism is grown on the infested soil, potatoes may be 
grown there again after several years. 

Disease-free seed. Great emphasis in the control of potato 
disease has been placed upon the necessity of using disease-free 
tubers ("seed"). Scab, Rhizoctonia, mosaic, dry rot, late blight, 
and other potato diseases may be carried into the soil by the tubers. 
Corn may carry within the kernel one or more disease-forming 
fungi. A number of the smuts are seed-borne. Anthracnose of 
beans is carried over from crop to crop largely in the seeds. Two 
general methods are adopted to secure disease-free seed, as follows: 
(1) seed selection, and (2) seed treatment. 

Disease resistance. Much progress in the control of plant 
diseases has been made through the breeding of disease-resistant 
strains. In fact, this is one of the most hopeful lines of attack in 
combating plant diseases. Resistance may be due to a structure 
which prevents entrance of the organism, or to a nutritional con- 
dition that does not supply the proper kind of food, or to other 
causes not well understood. 


No two plants are alike. They show variation. They may 
vary not only in such particulars as height, color, leaf shape, charac- 
ter of fruit, etc., but also in resistance to disease, or performance in 
some other direction. 

The wilt disease of cotton at one time threatened the cotton 
industry in the southern states. Progress in its control has been 
due to the development of resistant varieties. Tests have shown 
that some varieties of cotton are several hundred times more 
resistant to the disease than others. Kanred wheat is a variety 
relatively resistant to black stem rust. Kieffer and Mclntosh 
pears are relatively more resistant to bacterial blight (fire blight) 
than the Bartlett variety. Early Crawford and Elberta peaches 
are more resistant to brown rot of stone fruits than are such varie- 
ties as Triumph and Alexandra. 

Sanitation. This includes destruction or removal of diseased 
tissues, the burning of refuse, and soil treatment. 

Practical control of apple and pear blight, a bacterial disease, 
is brought about by pruning out infected twigs and smaller 
branches, and by scraping off all diseased tissue of the large 
branches and main trunk. The instruments employed are also 

In the case of black knot of plums and cherries, the developing 
knots should be pruned out as soon as they appear in the spring, 
and thus prevent the spread of spores which would develop on 
these swollen areas. Several prunings a season may be necessary 
to remove the diseased tissue. 

The large, swollen, smut masses that develop on corn are well 
known as the source of clouds of spores which are readily blown 
by the wind and may infect other plants. Stalks affected with 
smut should be cut out and burned before the spores mature. 

Crop residues and refuse often carry a disease from one season 
to the next. For example, if smutted corn is thrown on the 
manure heap, the spores may be carried to the field and become a 
source of infection. 

As a measure of control in onion mildew, the tops of diseased 
plants should be destroyed. If they are left on the land or returned 
to it in the manure, infestation of the new crop will occur. 

The mummied fruits characteristic of the brown rot of stone 



fruits should be knocked from the trees, and together with those 
on the ground raked together and burned. These mummied 
fruits are the principal source of infection the following year. 

Many other plant diseases are effectually controlled or held in 
check by the destruction of crop residue or refuse. 

A number of disease-forming organisms live over in soil, either 
in the vegetative stage or spore stage. For example, damping- 
off fungi, which are often prevalent in potting beds, in greenhouses, 
in seedling nurseries, and sometimes even in fields, live from year 
to year in the soil, attacking the plants in the seedling stage. 
Fungi in the soil may be destroyed by steam sterilization of the 
soil and by drenching it with formalin. Club root of cabbage, 
a disease which attacks 
a number of cruciferous 
plants such as cauli- 
flower, turnip, rutabaga, 
Brussels sprouts, radish, 
and other mustards, may 
be prevented or checked 
by applying lime to the 
soil, at the rate of about 
100 bushels per acre. 

Application of fungi- 
cides. The spraying or 
dusting of plants with 
chemicals is now a com- 
mon method of 

FIG. 221. Distributing sulphur by means o* 
the aeroplane, in the control of fungous dis- 
eases. The aeroplane is now used to distrib- 
ute various insecticides and fungicides. (From 
California Agricultural Experiment Station 
Bulletin 511.) 

trolling many fungus diseases. Twenty-five years ago, however, 
the practice was little used. In the use of most fungicides the object 
is to cover the surface of the plant with a chemical which will pre- 
vent the germination and growth of spores that may already be on 
the surface or light on it later. The spore itself may absorb suffi- 
cient of the poison on the plant surface to kill it; or when the spore 
germinates, and sends out a slender tube, if there is a poisonous 
chemical in its path, and this is absorbed, growth is prevented. 
Thus sprays and dusts are preventives rather than cures. 

Of course, the fungicide employed must not be injurious to the 
host. Fortunately, the thick cuticle which covers the surfaces 


of twigs, leaves, and fruits usually prevents the absorption of 
sufficient poison to injure the host, providing the fungicide is 
properly made. 

The common fungicides in use today are Bordeaux mixture, 
copper sulphate, ammoniacal copper carbonate, lime sulphur, 
flowers of sulphur, corrosive sublimate, and formalin. 

Application of insecticides. Insect pests with biting mouth- 
parts, as cabbage caterpillars, may usually be destroyed by spray- 

4 ing or dusting on a so- 

called stomach poison 
which is eaten by the in- 
sect as it feeds on the tis- 
sues of the plant. Insects 
with piercing mouth- 
parts, as plant lice and 
scale bugs, are not affect- 
ed by ordinary poisons 
placed on the surface of 
plants. These pests are 
destroyed by spraying with 
contact poisons. Examples 
of stomach poisons are Pa- 
ris green and lead arsen- 
ate; examples of contact 
poisons are tobacco infu- 
sion, lime sulphur wash, 
and kerosene emulsion. 

Tree surgery. The 
work of various wood- 
destroying fungi, and me- 

FIG. 222. Improper removal of a limb 
may result in decay that is carried far into 
the tree. In the above, a stub was left, 
which does not heal over; the stub finally 
decays, and falls out. The dark-colored 
portion represents decayed tissue. (Re- 
drawn from Solotaroff, in Shade Trees in 
Towns and Cities.) 

chanical injuries of differ- 
ent sorts, may necessitate special surgical methods to prevent the 
destruction of trees. The underlying principles in these methods 
are (1) the removal of all diseased or dead tissue, so as to secure a 
fresh surface, thus permitting the development of wound cork; (2) 
the sterilization of the exposed surface; (3) the covering of the sur- 
face with some material which will not allow the entrance of fungi. 
If fungi invade the tissue and cause decay, it is necessary to 


remove some live tissue beyond that which is clearly dead. Com- 
mon materials used to sterilize and cover the exposed surface are 
commercial creosote, asphaltum, tar, and Bordeaux paste. 


Many hundreds of plants are known to be poisonous to man 
and domestic animals, and many more hundreds are under sus- 
picion. The annual toll of human lives in the United States due 

FIG. 223. Edible mushrooms. It is often difficult to tell whether mushrooms 
are edible or poisonous. 

to eating " toadstools " and the roots and berries of various seed 
plants is considerable; and the yearly losses of livestock, particu- 
larly on the western ranges, due to poisonous plants amounts to 
several millions of dollars. 

From an early day the different Indian tribes have been skillful 
in preparing arrow poisons. A considerable number of different 
species of plants have furnished poison for arrow tips. The 
Egyptians were familiar with such poisonous plants as hyoscyamus, 
aconite, and conium. They also knew prussic acid, which they 
extracted from peach pits. 


Forage poisoning in livestock is thought to be caused by various 
fungi. Ergotism is a disease of livestock caused from eating 
grasses which contain ergot, a fungus. 

A number of the fleshy fungi are poisonous. Although there is 
no botanical difference between " mushrooms " and " toadstools/' 

FIG. 224. Deadly A manita, 
a poisonous mushroom. 
Note the swollen base, and 
the ring on the stem. The 
gills bear white spores. 

FIG. 225. Among our poisonous plants 
is the white snakeroot of our pastures. 
It not only poisons domestic animals 
that eat it, but it has been found to 
cause milk sickness in man. 

the former name is commonly applied to those believed to be 
edible, and the latter to those thought to be poisonous. 

Nearly all the deadly poisonous fleshy fungi are species of the 
genus Amanita. This is a group of mushrooms with gills. Some of 
the species of Amanita have white caps, others have bright orange, 
red, or yellow caps; but in all the gills are white. This deadly 



poisonous group is distinguished by the following combination of 

1. White gills. 

2. A " ring " on the stem. 

3. A cup at the base the so-called " death cup." 

When a mushroom shows these three features, it should be 
avoided. One should not de- 
pend upon the various rules-of- 
thumb for detecting poisonous 

A large number of plants 
are known to be poisonous 
from the presence of prussic 
or hydrocyanic acid. This 
acid is known to be one of the 
most deadly poisons, and it 
results from the presence in 
the plant of what is known as 
a glucoside, which must be 
acted upon by a ferment. A 
large number and a great 
variety of plants contain a 
glucoside. Plants conspicuous 
in this class are cherry, peach, 
and other stone fruits, sor- FIG. 226.The whorled milkweed is an 
ghum, kafir corn, and Johnson important stock poisoning plant. 

Plants chiefly responsible for injury and losses of livestock on 
the ranges of the western United States are as follows : larkspurs 
(Delphinium), aconite (Aconitum), death camas (Zygadenus), 
lupine (Lupinus), loco weed (Aragallus and Astragalus), water 
hemlock (Cicuta), milkweed (Asclepias), horsetail (Equisetum). 

A number of plants are poisonous to the touch, causing skin 
diseases. Chief of these is the poison oak or poison ivy. No 
plant of the United States is more popularly recognized as harmful 
to man than this. 


One of the most deadly poisonous plants is the water hemlock 
(Cicuta). Its poisonous principle is in an aromatic, oily fluid 
which is found chiefly in the roots; not infrequently children eat 
the roots, mistaking them for radish, parsnip, and other edible 
roots. The poison hemlock (Conium maculatum), closely related 
to water hemlock, is also a deadly poisonous plant, which has 

FIG. 227. Water hemlock, an extremely poi- 
sonous plant. 

FIG. 228. A portion of the roc 
stock and aerial stem of wal 
hemlock, a very poisonous plai 
cut in lengthwise section. Obser 
the narrow parallel compa; 
ments, a feature which enables o 
to identify this plant. 

become naturalized in the United States. It is a native of Europe. 
It is the plant a decoction of which was administered to Socrates 
and caused his death. 


It is now known that pollen is one of the chief causes of hay 
fever or bronchial asthma. The plants which give the most 



trouble are principally weeds, and certain wind-pollinated trees 
which produce an abundance of pollen. Listed among the worst 
hay-fever plants are the following: ragweeds, wormwoods, pig- 
weeds, Russian thistle, many grasses, including corn, oats, timothy, 
and wheat, and such trees as oak, 
black walnut, cedar, elms, and 

The amount of pollen given off 
by certain plants is enormous. It 
has been computed that a single 
plant of ragweed (Ambrosia trifida) 
gave off between 8 A.M. and 1 P.M. 
in one day approximately 8,000,000,- 
000 pollen grains. These are carried 
by the air currents, and reach the 
membranes of the respiratory tract. 
If the individual receiving them is 
" sensitive " to the particular pollen, 
he develops hay fever. 


FIG. 229. One of the rag- 
weeds, a plant which produces 
an abundance of pollen. It is a 
well-known hay fever plant. 

Truck Crop Plants, by HENRY ALBERT 
by the McGraw-Hill Book Company, New 
York, 1928. 538 pages and 98 illustrations. 
The authors express the hope that the "book 
will satisfy more than the purely economic 
side. There should be happiness and enjoy- 
ment in the growing and handling of vegetable crops. Moreover, the gar- 
dener working with various crops or plants and knowing something about their 
behavior, structure, manner of growth, and relation to their environment is 
usually more completely satisfied than one working with the same crops and 
knowing nothing of the secrets which they hold." 

The Tropical Crops, by O. W. BARRETT, published by the Macmillan 
Company, New York, 1928. 445 pages, 24 plates. Forty per cent of the 
earth's surface is in the tropics. The crops of the tropics now " supply a very 
prominent part of the international trade in foodstuffs, fibers, and industrial 
oil materials." This is a very readable account of such topics as tropical 
field practices and conditions, living conditions for the tropical planter, and 
the most important tropical crops. 

The Standard Cyclopedia of Horticulture, by L. H. BAILEY, published by 


the Macmillan Company, New York, 1922. In six large volumes, a total of 
3639 pages and 4056 illustrations. It is described as a "discussion, for the 
amateur, and the professional and commercial grower, of the kinds, char- 
acteristics, and methods of cultivation of the species of plants grown in the 
regions of the United States and Canada for food, for ornament, for fancy, 
for fruit, and for vegetables; with keys to the natural families and genera, 
descriptions of the horticultural capabilities of the states and provinces and 
dependent islands, and sketches of eminent horticulturists. 

Manual of Poisonous Plants of the United States, by L. H. PAMMEL, 
published by the Torch Press, Cedar Rapids, Iowa, 1911. A large book of 977 

FIG. 230. The golden rod is a hay fever plant. However, as its pollen is 
sticky, it is not a serious offender. 

pages and 458 illustrations. It contains chapters on bacterial poisons, derma- 
titis, forage poisoning, poisoning from fungi, poisoning from various flowering 
plants, fish and arrow poisons, classification of poisons, symptoms and anti- 
dotes, the production of poisons in plants, chemistry of alkaloids, glucosides, 
etc., and a catalogue of the most important poisonous plants of the United 
States and Canada, and also a complete bibliography of poisonous plants. 

Manual of Plant Diseases, by F. D. HEALD, published by the McGraw- 
Hill Book Company, New York, 1926. 891 pages, 272 illustrations. This 
book presents a view of the whole field of plant pathology. It deals with 
symptoms of disease in plants, diseases due to deficiencies of food material in 
the soil, diseases due to excesses of soluble salts in the soil, diseases due to 


unfavorable water relations, diseases due to improper air relations, diseases 
due to high and to low temperatures, diseases due to unfavorable light rela- 
tions, diseases due to manufacturing or industrial processes, diseases due to 
control practices, virus diseases, and the great number of diseases due to 
parasitic organisms such as bacteria, slime molds, rust fungi, smut fungi, etc. 

One Thousand American Fungi, by CHARLES MC!LVAINE, published by 
the Bobbs-Merrill Company, Indianapolis, 1912. 749 pages, and 216 illus- 
trations, many of which are full-page colored plates. It treats of toadstools 
and mushrooms, edible and poisonous, and tells how to select and cook the 
edible, and how to distinguish and avoid the poisonous. 

Plants Useful to Man, by W. W. ROBBINS and FRANCIS RAMALEY, pub- 
lished by Blakiston, Philadelphia, 1933. 428 pages, 241 illustrations. This 
book furnishes a background of knowledge of the world's commercial plant 
products both for students of botany and for those whose interests are in the 
fields of geography, economics, and agriculture. It includes a discussion of 
common crop plants of orchard, garden, and field, of the more usual orna- 
mentals, and also of plants in tropical and subtropical countries which yield 
such materials as tea, coffee, spices, drugs, fibers, and tropical fruits. 

The Microscopy of Vegetable Foods, by A. L. WINTON, published by John 
Wiley and Sons, New York, 1916. 701 pages, 589 illustrations. In addition 
to a description of the microscopic structure of all important food products 
and the organs from which they are derived, there is special mention of methods 
to be employed in the detection of adulteration and the diagnosis of mixture. 

Our Edible Toadstools and Mushrooms, by W. HAMILTON GIBSON, 
published by Harper Brothers, New York, 1895. 337 pages, with 30 colored 
plates and 57 other illustrations. "A selection of thirty native food varieties 
easily recognizable by their marked individualities, with simple rules for the 
identification of poisonous species/' 

Botany of Crop Plants, by W. W. ROBBINS, third edition, published by 
Blakiston, Philadelphia, 1931. 639 pages, 269 illustrations. 

Shade Trees in Towns and Cities, by WILLIAM SOLOTAROFF. John Wiley 
and Sons, New York, 1911. 287 pages, 45 plates and 35 figures. This book 
deals with shade trees, their selection, planting, and care as applied to the art of 
street decoration; their diseases and remedies; their municipal control and 

Economic Plants, by ERNEST E. STANFORD. D. Appleton-Century 
Company, New York, 1934. 571 pages, 376 figures. "A brief survey of 
several of the more important groups of plants and plant products utilized by 
the human race." 


Numbers in bold-face indicate pages carrying illustrations. 

Absorption, 3; conditions which in- 
fluence rate of, 54; how plants are 
fitted for, 249; of roots, 47; of 
water, 125; process of, 51 

Acacia Senegal, 353, 358 

Achene, 282, 283 

Aconite, 381 

Aconitum, 381 

Acorus calamus, 358 

Acquired immunity, 98 

Adventitious roots, 46, 47 

Aeciospores of stem rust, 107 

After-ripening, 134 

Agarics, 17 

Agave, 348, 351 

Age: of seeds, 133; of trees, deter- 
mining, 142 

Agents which disperse pollen, 157 

Aggregate fruits, 282 

Air: bacteria in, 100; in the soil, 229; 
movement of, 238; relation of 
plants to, 236; temperature of, 227 

Albinism in corn, 310 

Aleurone layer of wheat grain, 36 

Alfalfa, 347; flowers of, 170; pol- 
lination of, 170 

Algae, 11; different groups of, 12; 
different kinds of, 11, 14 

Alga-like fungi, 110 

Alkaloids, 71 

Almond, 342 

Amanita, 380 

Ambrosia trifida. 383 

Ammonifying bacteria, 232, 233 

Ananas sativus, 342 

Anatomy of leaf, 59 

Anchorage of roots, 47 

Angiosperms, 327, 329 

Animals: dispersal of fruits and seeds 

by, 285; interrelation of to plants, 

238; pollination by, 274 
Anise oil, 354 

Annual rings of growth, 139, 141 
Annuals, 76, 336 
Anther, 160, 151; cross-section of 

mature, 153 
Antheridium: of fern, 176; of moss, 


Antitoxins, 98 
Apple: flower of, 167, 168; rust of, 


Approach grafting, 196, 199 
Apricot, 340 
Aquarium, balanced, 82 
Arachis hypogoea, 343, 353 
Aragallus, 381 
Archegonium: of fern, 176; of moss, 


Aristolochia, 273 
Artificial cross-pollination, 306 
Asclepias, 381 
Ascomycetes, 110 
Ascophyllum, 13 

Asexual reproduction, 181, 182 
Asparagus, 343; flowers and fruit of, 


Assimilation, 3, 30; of food, 79 
Astragalus, 381,353 
Atropa belladonna, 357 
Atropine, 71, 357 




Autophytes, 81 
Avena, 329 
Avocado, 340 


Bacteria, 15, 110; ammonifying, 232, 
233; diseases due to, 372; forms of, 
16; in air, 100; in milk, 100; in 
relation to soil fertility, 232; in 
water, 99; nitrifying, 232, 234; 
nitrogen fixing, 96; plant diseases 
caused by, 104 

Bacteria and molds: in decay, 94; 
laboratory study of, 99 

Balanced aquarium, 82 

Bamboo, 352 

Bambusa polymorpha, 362 

Banana, 340 

Barberry, 108 

Bark, 63, 64, 139 

Barley, 335 

Barriers, 297 

Basidiomycetes, 112 

Basidium fungi, 112 

Bean: fruit of Lima, 284; seeds, 
stages in germination of, 117 

Bees: as pollinating agents, 159; in 
orchards, 160 

Belladonna, 357 

Berry, 281 

Beta vulgaris, 344, 359 

Beverage plants, 344 

Biennials, 76, 366 

Binomial, 329 

Birds, agents in weed dissemination, 

Blackberry, 340 

Black locust, fruit of, 284 

Black mosses, 19 

Black pepper, 345 

Blade of leaf, 57 

Blanching: of cauliflower, 222; of 
celery, 221 

Bluegrass, germination of seeds of, 

Blue-green algae, 11, 12 

Blue and green molds, 16, 110 

Borecole, 343 

Botany, a definition, 6 

Botulinus poisoning, 90 

Botulism, 90 

Box elder stem, growth in diameter 
of, 64 

Brassica napus, 353 

Brazil nut, 342 

Bread making, use of yeast in, 89 

Bread mold, 101; asexual reproduc- 
tion in, 183; collection of, 101; 
gametic reproduction in, 102; life 
cycle of, 103; nature of, 101; 
sporangia of, 183; spore formation 
in, 183 

Bridge grafting, 196 

Brown, Robert, 28 

Brown algae, 12 

Brussels sprouts, 343 

Bryophytes, 326 

Buckwheat, 335 

Bud: definition, 137; flower, 136; 
flower of apricot, 161; grafting, 
196; lateral, 136; mutations, 314; 
opening of, 137; structure of, 136; 
terminal, 136 

Budding, 196, 197, 198 

Bulb, 62, 186 

Bulbel, 187 

Bulblet, 187 

Burdock, fruit of, 286 

Butternut, 342 

By-products: derived from plants, 
358; of food building, 67 

Cabbage, 343 

Cacao, 344, 353 

Cactus, stages in development of, 126 

Caffein, 10, 71 

Calamus, 358 

Calcium, 42; r61e of in plants, 74 

Callus, 190, 191 

Calyx, 151 



Cambium, 63, 64, 139, 141 

Camphor, 358 

Canada thistle, 367; rhizome of, 62 

Cane sugar, 35, 67 

Cannabis, 348, 349, 359 

Cap of mushroom, 18 

Capsicum, 346 

Capsule, 283; of moss, 19 

Carbohydrates, 34, 40; manufacture 

of, 69 
Carbon: r61e of in plants, 74; source 

of, 42 
Carbon dioxide: process of intake, 61 ; 

test for presence of, 89 
Carboniferous Age, 5 
Carnivorous plants, 250, 252 
Carotin, 28 
Carrion fungi, 17 
Carrot, 343 
Caryopsis, 283 
Castor oil, 353, 358 
Catkins of walnut, 167 
Cauliflower, blanching of, 222 
Cayenne pepper, 346 
Cedar oil, 353 

Celery, 343; blanching of, 221 
Cell, 3; division of, 302, 303; division 

of in formation of eggs and sperms, 

166; sap, 25; structure of, 24; 

wall, 24, 119 
Cells: and tissues, different kinds of, 

27; different kinds of, 28; growth 

of in root tip, 119; structure of, 24 
Cellulose, 25, 35, 37, 70 
Century plant, 351 
Cereals, 335 
Certified milk, 91 
Chaulmoogra tree, 367 
Chemical substances found in plants, 


Chemotropism, 247 
Cherimoya l 341 
Cherry, 340 
Chestnut, 342 
Chives, 343 

Chlorophyll, 28, 42, 61; effect of light 
on, 223; extraction of from leaves, 
222; role of in food building, 67 

Chloroplasts, 26, 61 

Chlorosis, 374 

Chocolate, 344 

Chokecherry, flower of, 169 

Chromosomes, 302, 303, 304 

Chrysanthemum, mutation in, 316 

Chrysanthemum indicum, 300 

Cicuta, 381, 382 

Cinchona, 10, 71, 356 

Cinnamomum camphora, 358 

Cinnamon, 345 

Citric acid, 71 

Citrus limonia, 354 

Cladophora, 13 

Classes of plants, 327 

Classification of plants, 321, 323, 333 

Cleft grafting, 193, 194 

Cleistogamous flowers, 157, 158 

Climbing stems, 62 

Close pollination, 271 

Clove oil, 353 

Cloves, 346 

Club mosses, 21, 326 

Club wheats, 335 

Coal, 351 

Coal Age, 5 

Coconut, 342, 351; oil, 353; palm, 

Cocos nucifera, 345, 353 

Coffea arabica, 344, 346 

Coffee, 344, 346 

Coffee berry, 10 

Coleus, variegated leaf of, 67 

Collenchyma of stems, 63 

Columbine flower, 272 

Common barberry in life history of 
wheat rust, 107, 108 

Communities, plant, 257, 260, 262 

Companion cells, 63 

Compass plant, 217 

Complete flower, 162 

Compositae, 169 



Composite flower, 169, 279 

Compound leaf, 58 

Conidiophore, 16 

Conidiospore, 16 

Coniwn maculatum, 382 

Conjugation in bread mold, 103 

CorchoruSj 351 

Cork, 63, 142, 352; cambium, 142; 

cells, 27 
Corm, 62, 187 
Corn, 334, 335, 359; grain of, 336; 

pistillate inflorescence of, 166; 

seedling of, 46; smut, 17; stem, 

structure of, 63; types of, 336 
Cornstarch, 359 
Corolla, 151 

Cortex of roots, 48; of stems, 63, 64 
Cotton, 348; principal varieties of, 


Cottonseed oil, 353, 358 
Cottonwood: staminate flowers of, 

158; twig of, 136 
Cotyledons, 117, 118, 125, 328 
Crop rotation, 96, 375 
Cross-fertilization, 306 
Cross-pollination, 271, 274 
Crown gall, 105 
Cruciferae, 165, 166 
Currant, 340 
Cutin, 255 
Cuttings: leaf, 192; propagation by, 

188; root, 189, 191; stem, 189, 190, 


Cycad, 296 
Cyas revoluta, 296 
Cycle of nitrogen in nature, 94 
Cyperaceae, 328 
Cypress, knees of, 260 
Cytoplasm, 24, 25, 28, 119 


Date, 340; paim, 341 
Death camas, 381 

Decay caused by bacteria and molds, 

Dehiscent fruits, 282 

Delay in the germination of seeds, 133 

Delphinium, 381 

Denitrification, 235 

Deodar, 23 

Dependent plants, 81 

Determiners, 304 

Dewberry, 340 

Dextrin, 353, 359 

Diastase, 78 

Diclinous flowers, 278 

Dicotyledons, 328 

Differentiation, 116, 119, 331 

Diffusion, 51, 52 

Digestion, 3; definition of, 78; of 
foods, 76; of starch, 79; of stored 
foods in seeds, 126 

Digitalin, 356 

Digitalis, 356 

Dihybrids, 319 

Dioon, 296 

Disease: principles of control, 374; 
resistance, 375 

Diseases due to rust fungi, 106 

Disk flowers, 169, 171 

Dispersal: by animals, 285; by pro- 
pulsion, 285; by water, 285; of dry 
fruits, 282; of fleshy fruits, 280; 
of fruits and seeds, 285, 286; of 
pollen, 156 

Division, propagation by, 187 

Dockage caused by weeds, 363 

Dodder, 112, 113 

Dominance, 311 

Dominant characters, 311 

Dormancy of weed seeds, 364 

Double fertilization, 147 

Double flowers, 170 

Downy mildews, 15 

Drupaceae, 167 

Drupe, 280, 281 

Duck meat, 150 

Duck weed, 150 

Dunes, succession in, 265 

Durum wheats, 335 


391 . 


Ear of corn, 163 
Ecology, 243 
Economic importance of plants to 

man, 334-385 

Egg, 153; cell, 118; nucleus, 155 
Eggplant, 343 
Elaeis guineensis, 353 
Elements, role of in plant nutrition, 72 
Elodea, 29, 53; leaf of, 25, 26; living 

protoplasm in cells of, 29 
Embryo, 119, 121, 155; growth of, 

118, 128; sac, 146, 153; structure 

of, 118 
Emmer, 335 
Elndosperm, 146, 336; of wheat grain, 


End-products of food building, 67 
Environment, 204, 243 
Enzymes, 78 
Epidermis: of geranium leaf, 59; of 

stems, 63, 65 
Epiphytes, 249 
Equisetum, 381 
Erect stems, 62 
Ergotism, 380 
Essential plant oils, 71 
Eugenia aromatica, 346, 353 
Evening primrose, opening of flowers 

of, 220 
Excretion, 30 

Fagopyrum vulgar e, 360 

Fats, 34, 36, 40; and oils, 70; test for, 


Fehling's solution, 41 
Fern: fronds of, 177; leaf of, 145; 

life history of, 176; prothallia of, 

178; sori of, 178; stag-horn, 21; 

walking, 182 
Ferns, 326; and allies, 20; and cy- 

cads, 295; gamete production in, 

175; how they reproduce, 175; 

spore production in, 175 

Fertilization, 118, 146, 153, 165, 202; 
double, 147 

Fiber plants, 348 

Fibers, 27; bast, 351; microscopic 
examination of, 351 

Fibrous root system, 43 

Ficus elastica, 354 

Fig, 340; pollination of, 241 

Filament, 150, 151 

Filbert, 342 

Fire blight, 105 

Fission, reproduction by, 182 

Flax, 348, 349; seed of, 358 

Flower, 150; apple type of, 167; 
bud, 136; bud of apricot, , 151; 
complete, 162; composite type of, 
169, 279; diagram of, 150; dicli- 
nous, 278; essential parts of, 277; 
grass type of, 165; incomplete, 162, 
163; irregular, 278; legume type, 
167; lily type of, 163; mustard 
type of, 165, 167; of apple, 168; 
of mustard, 167; of pea, 169; of 
sour cherry, 168; of wheat, 165; 
parts of, 151; plum type of, 167; 
regular, 278; rose type of, 166 

Flowering plants, how they repro- 
duce, 160 

Flowers: and insect visitation, 272; 
animal pollinated, 274; cleistoga- 
mous, 157, 158; different types 
of, 162; disk, 169; double, 170; 
hermaphroditic of strawberry, 173; 
imperfect, 172; monoclinous, 278; 
of chokecherry, 169; of Jerusalem 
artichoke, 171; of maize, 278; 
open types of, 278; pollen recep- 
tion of, 277; ray, 169; specialized, 
279; types of with reference to 
pollination, 278; wind pollinated, 

Fluctuations, 313 

Follicle, 283 

Food: assimilation of, 79: building 
processes of, 66; energy, 6; for 



livestock, 347; materials, putrefac- 
f action of, 93; plants, 335; use 
made of by plant, 68 

Foods, 339; kinds of stored, 77; 
movement of in plants, 75; nature 
of in plants, 40; preservation of, 
91; process of digestion of, 77; 
reserve, 70; storage and digestion 
of, 76 

Forest fire destruction, 369 

Forms of the plant body, 10 

Forsythia, 314 

Fossils, 293, 294 

Frond, 20; of fern, 177 

Fructose, 35, 70 

Fruit, 147; causes of failure to set, 
171; definition of, 279; dispersal 
of, 286; of black locust, 284; of 
burdock, 286; of Lima bean, 284; 
of pineapple, 283; of raspberry, 
282; of white ash, 286 

Fruits, 339; aggregate, 282; defini- 
tion of, 287; dehiscent, 282; dif- 
ferent types of, 287; dispersal, 279; 
dry, how fitted to dispersal, 282; 
fleshy, how fitted to dispersal, 280; 
indehiscent, 282; means of dis- 
persal, 285; multiple, 282; pome, 
280; stone, 281 

Functions of roots, 47 

Fungi, 14; diseases caused by, 372; 
principal groups of, 109 

Fungicides, application of, 377 


Gamboge, 353 

Gametes of bread mold, 102 

Gametophyte: of fern, 176; of moss, 


Garcinia handuryu, 353 
Garden beet, 343 
Garlic, 343 

Gaultheria, 346; procurnbens, 354 
Gemmae of liverworts, 181, 183 
Genes, 304 

Gentian, 157 

Genus, 328 

Geotropism, 247 

Geranium leaf, 57; epidermis of, 69; 
response to light, 218 

Germinating: seeds of mustard, 126; 
stages of wheat, 123 

Germination: as influenced by oxy- 
gen, 124; as influenced by tem- 
perature, 122; conditions neces- 
sary for, 121; nature of seed, 121; 
of bean seed, 117; of pollen grain, 
152; of pumpkin seed, 124; proc- 
ess of, 125; of seeds, delay in, 133 

Gill fungi, 17 

Ginger, 346 

Ginkgo, 298 

Girdling of stems, 75 

Gladiolus, corm of, 62 

Gloeocapsa, 11, 12 

Glucose, 35, 67, 70, 359; manufac- 
ture, 41 

Glume, 165 

Goldenrod, 384 

Gooseberry, 340 

Gossypium, 348, 353 

Grafting: approach, 199; bridge, 
196; cleft, 193, 194; kinds of, 196; 
propagation by, 194; saddle, 196; 
side, 196; tongue or whip, 194 

Grain: of corn, 336; of wheat, in 
section, 36 

GraminaleSy 329 

Gramineae, 328, 329 

Grape, 338, 339, 340, 361; sugar, 35 

Grapefruit, 340 

Grass type of flower, 165 

Grasses, habit of growth of, 22 

Gravity, effect of: upon primary 
root, 246; upon stem, 246 

Green: algae, 12; plants, 39; plants, 
converters of solar energy, 5 

Growing point, 144 

Growth, 3, 30; in diameter of stems, 
64, 138; in length of stems, 135; 



of cells in a root tip, 119; of 
embryo, 118, 128; of leaves, 144, 
145; of plant cell, 119; of plants, 
116-148; of roots, 143; of seeds 
and fruits, 145 

Guard cells, 59, 60, 208 

Guava, 341 

Guayula, 354 

Gum: arabic, 353, 357; tragacanth, 

Gums, 71; vegetable, 353 

Guncotton, 358 

Guttapercha, 354 

Gymnosperms, 22, 327 


Habitat, 244 

Haematoxylin campechianum, 353 

Halophytes, 365 

Hard: seeds, 126, 134; wood, 142 

Haustoria, 113 

Hay fever plants, 382 

Hazelnut, 342 

Heartwood, 139, 140 

Hemp, 348, 349, 350, 359 

Herbaceous: stem of sunflower, struc- 
ture of, 62; type of stem, 61 

Herbs, 62, 325 

Heredity, a definition, 301 

Hermaphroditic flowers of straw- 
berry, 173 

Hesperidium, 281 

Hevea brasiliensis, 354 

Hickory nut, 342 

Honey bee, legs of, 272 

Hooke, Robert, 24, 28 

Hordeum, 329 

Horsetail, 20, 21, 326, 381 

Host, 14 

Humus, 223 

Hybrid, 306; vigor, 321 

Hybridization, 306 

Hydrophytes, 267 

Hydrotropism, 247 

Hypha, 16 

Hyphae, 102; of bread mold, 101 
Hypocotyl, 125; of bean, 117 

Immunity, 98 
Inarching, 196, 199 
Incomplete flower, 162 
Indehiscent fruits, 282 
Independent plants, 81 
India rubber tree, 354 
Indian compass plant, 252 
Indian pipe, 111, 113 
Indusium of ferns, 175 
Industrial plants, 348 
Insect pollination, 276 
Insecticides, application of, 378 
Insects, diseases caused by, 374 
Internode, 136 

Iron, 42; role of in plants, 74 
Irregular flowers, 278 

Jerusalem artichoke, 343; flowers of, 

Jute, 351 


Kalaw, 357 
Kale, 343 
Keel, 169 
Kelp, 12 
Key fruit, 284 
Knees of cypress, 260 
Koch, Robert, 87, 98 
Kohlrabi, 343 
Kumquat, 340 

Larkspur, 381 

Lateral buds, 136 

Latex, 71 

Layering, propagation by, 192 

Leaf: anatomy of, 59; blade of 
wheat, 22; compound, 58; cross- 
section of, 60; cuttings, 192; loss 
of water from, 208, 209; mosaic, 



216; of Elodea, 26; of fern, 145; 
of geranium, 57,; position and light, 
216; sheath of wheat, 22; simple, 
58; veins, 60, 65 

Leaflets, 58 

Leaves: different types of, 312; epi- 
dermis of, 59; how they grow, 144, 
145; loss of water from, 207; of 
Venus' flytrap, 251; structure of, 
58; terms describing, 58; types 
of, 56, 57, 58, 250 

Leek, 343; inflorescence of, 164 

Leeuwenhoek, 98 

Legume, 282; type of flower, 167 

Leguminosae, 167 

Lemma, 165 

Lemna, 150 

Lemon, 340, 346; oil, 354 

Lenticels, 236 

Lettuce, 343 

Leucoplasts, 27 

Lichen, 19; reindeer, 110 

Lichens, 111, 262, 263 

Life cycle of bread mold, 103 

Light: and leaf position, 216; as 
influencing the movement and 
position of plant organs, 216; 
duration of, 218; effect upon form 
of plant, 245; energy, 6; intensity 
of, 218; its influence on size, form 
and structure, 216; quality of, 218, 
223; relation of to plant life, 215; 
response of leaves to, 216; response 
of plants to, 218, 219, 245; r61e of 
in food building, 66 

Lilium: candidum, bulbel in, 187; 
grandiflorum, 164; tigrinum, bulb- 
lets in, 187 

Lily type of flower, 163 

Lime, 340 

Linnaeus, 330 

Linseed oil, 353 

Linum, 348; iwitatissimum, 349, 353 

Lipase, 79 

Lister, 99 

Live oak, 23 

Liverworts, 19, 20, 326; gemmae of, 

181, 183 
Loco weed, 381 
Lodging of cereals, 73 
Loquat, 340 
Lupine, 381 
Lupinus, 381 


Mace, 346 

Magnesium, 42; role of in plants, 74 

Maidenhair tree, 298 

Maize oil, 353; types of, 336 

Mangosteen, 341 

Mangelwurzels, 347 

Mango, 340 

Manila fiber, 350, 351; hemp, 351 

Marchantia, 20 

Mass selection, 320 

Maximum temperature, 211; for 
germination, 122 

Meadow sage, pollination of, 239 

Medicago, 347 

Medicinal plants, 355 

Mendel, Gregor, 306 

Mendel's laws, 309; of heredity, dia- 
grams illustrating, 307 

Mentha, 346; piperitdj 354 

Menthol, 358 

Mesophyll of leaf, 57, 59 

Mesophyte, laboratory study of, 270 

Mesophytes, 258 

Micropyle, 146, 153 

Mildews, 15 

Milk: bacteria in, 100; certified, 91; 
how diseases may be spread by, 90; 
pasteurization of, 91; pasteurizer, 

Milkweed, 381 

Millet 335 

Mineral nutrients of the soil, 229 

Minimum temperature, 211; for ger- 
mination, 122 

Moccasin flower, 275 



Molds, 15; blue and green, 16, 110 

Monoclinous flowers, 278 

Monocotyledons, 328, 329 

Morning-glory, rhizome of, 62 

Morphine, 71 

Mosaic, 216, 248 

Moss, 179; antheridium of, 180; 
archegonium of, 180; asexual plant 
of, 179; gametophyte of, 180; life 
cycle of, 180; spore production in, 
179; sporophyte of, 180 

Mosses, 19, 262; and liverworts, 326; 
groups of, 19; how they repro- 
duce, 175 

Movement and position of plant or- 
gans as influenced by light, 216 

Mulberry, 340 

Multiple fruits, 282 

Musa textilis, 350, 351 

Mushroom: longitudinal section of, 
18; poisonous, 380; shaggy-mane, 

Mushrooms, 17, 18, 84, 111, 380; 
edible, 379 

Musk melon, 340 

Mustard: flower of, 167; germinat- 
ing seeds of, 125; type of flower, 

Mutants, 314 

Mutation, 313; bud, 314; in sun- 
flower, 315 

Mycelium, 102; of bread mold, 101 

Myristica fragrans, 346, 354 


Narcissus, bulb of, 62 
Natural: immunity, 98; selection, 


Nature of seed germination, 121 
Nectar glands, 160, 273 
Nectaries, 160, 273 
Nectarine, 340 
Net venation, 57 
Nicotine, 71 
Nitrates, 42, 73 

Nitrifying bacteria, 232, 234 

Nitrogen, 42; cycle in nature, 94, 96, 
235; fixation, 233, 234; fixing 
bacteria, 96; r61e of in plant, 73 

Node, 136 

Non-green plants: main characteris- 
tics of, 83; nutrition of, 81-115 

Nostoc, 11, 13 

Nucellus, 153; of wheat grain, 36 

Nuclei, sperm, 146 

Nucleus, 24, 28, 119; tube, 146 

Nut, 284, 342 

Nutmeg, 346; oil, 354 

Nutrition: of green plants, 39-80; 
of non-green plants, 81-115 

Nux-vomica, 358 


Oats, 335; spikelet of, 164 

Oil, first visible product of photo- 
synthesis, 67 

Oils, 36, 353 

Olea europea, 353 

Olive, 340; oil, 353 

Onion, 343; skin of , 26 

Opium, 355 

Optimum temperature, 211; for ger- 
mination, 122 

Orange, 340, 361 

Orchard heating, 213 

Orchid: flower, 274, 331; method of 
propagating, 185 

Orders, 327 

Organic acids, 71 

Organization and composition of 
plants, 9-38 

Organs, 3, 9, 30, 31 

Oryza sativa, 337 

Oscillatoria, 13 

Ovary, 150, 151 

Ovule, 153; structure of, 153 

Ovules, 146, 151 

Oxygen: and germination, 124; given 
off in photosynthesis, 67; r61e of in 
plants, 74 



Palaquium oblongifolia, 354 
Palisade: layer, 60; parenchyma, 


Palm, 344; oil, 353 
Palmately veined leaves, 57 
Papaver somniferum, 355 
Papaya, 340 
Parallel venation, 57 
Parasite, 14 
Parasites, 86, 252 
Parasitic plants which cause disease, 

97, 104 
Parsley, 343 
Parsnip, 343 

Parthemum argentatum, 354 
Partheiiocarpy, 155 
Parthenogenesis, 155 
Pasteur, Louis, 87, 98 
Pasteurization, 89; of milk, 91 
Pasteurizer for milk, 91 
Pathology of plants, 87 
Pea, flower of, 169 
Peach, 340; fruit of, 280 
Peanut, 342, 343; oil, 353, 360 
Pear, 340 

Peat bog, 261; mosses, 19 
Pecan, 342 
Pedicel, 160 

Pedigree culture, 319, 320 
Pelican flower, 273 
Pepo, 281 
Pepper, 345, 347 
Peppermint, 346; oil, 354 
Pepsin, 79 
Perennials, 76, 366 
Pericarp of wheat grain, 36 
Permanent wilting percentage, 224 
Persimmon, 341 
Petal, 160 
Petals, 151, 162 
Petiole of leaf, 57 
Petrified wood, 294, 296 
Phloem, 64, 139; fibers, 63; of 

stems, 63; parenchyma, 63 

Phoenix: dactylifera, 341; sylvestris, 

Phosphates, 42 

Phosphorus, 42 ; role of in plants, 73 

Photosynthesis, 32, 42, 67, 69; 
process summarized, 68 

Phototropism, 247 

Phy corny cetes, 110 

Pileus of mushroom, 18 

Pine: cones of, 328; four-year-old 
stem of, 139; wood, 141, 142 

Pineapple, 340, 342; fruit of, 283 

Pinnately veined leaves, 57 

Piper nigrum, 345, 347 

Pistachio, 342 

Pistil, 150, 151, 162 

Pistillate: flowers, 163; flowers of 
asparagus, 162; inflorescence of 
corn, 166 

Pitcher plant, 250, 262 

Pith, 63, 139, 140 

Pits, bordered, 27 

Plant: body of, 9; forms of, 10; cell, 
how it grows, 119; communities, 
257, 260, 262; communities, meso- 
phytic, 264; diseases, 371; dis- 
eases, causes of, 372; diseases 
caused by bacteria, 104; diseases 
caused by parasitic plants, 104; 
diseases due to smut fungi, 109; 
dyes, 353; ecology, 243; foods, 
nature of, 40; pathology, 87; 
skeleton, materials which compose, 
70; succession, 265 

Plants: and animals, interrelation of, 
238; carnivorous, 250; classifica- 
tion of, 321, 323, 333; dependent, 
81; development and improve- 
ment of, 290-322; economic im- 
portance of, 334; fiber, 348; four 
great groups of, 326; growth of, 
116-148; hay fever, 382; how 
they interfere with man, 361; in- 
dependent, 81; industrial 348; in 
what ways they have changed, 291; 



medicinal, 355; nutrition of non- 
green, 81-115; poisonous, 379; 
reproduction of, 149-203; shade and 
sun, 222 

Plasmolysis, 53 

Plastids, 25, 28 

Plum, 340; type of flower, 167 

Plumule, 117; 125 

Pod, 284 

Poison: hemlock, 382; oak, 381 

Poisonous plants, 379 

Pollen, 151; agents which disperse, 
157; dispersal of, 156; grain, 152; 
different kinds of, 156; germina- 
tion of, 152; longevity and viabil- 
ity of, 161; quantity of, 157; tube, 
146; where produced in the flower, 

Pollination: artificial, 321; close, 
271; cross, 271; how plants are 
related by structure to the process 
of, 271; immediate effect of, 161; 
of fig, 241; of meadow sage, 239; 
self, 271, 277; types of flowers with 
reference to, 278 

Polypodium, 253 

Pomaceae, 167 

Pome fruits, 280 

Pond scum, 13 

Poppy, 355 

Pore fungi, 17, 18 

Potassium, 42 ; role of in plants, 74 

Potato, 343, 361; bulb of, 62; stor- 
age cells of, 26; tuber of, 35; 
wild, 317 

Powdery mildews, 15 

Prickly pear, 259 

Primary roots, 47 

Pronuba moth, 240 

Prop roots, 47 

Propagation: by cuttings, 188; by 
division, 187; by grafting, 194; 
by layering, 192; by layers, 192; 
by separation, 186; by stolons or 
runners, 193; by suckers, 193; of 

orchids, 186; of plants artificially, 
184; of strawberry, 188 

Propulsion, dispersal of fruits and 
seeds by, 285 

Prostrate stems, 62 

Prothallium of ferns, 175, 176, 178 

Proteins, 34, 36, 40, 70; test for, 41 

Protococcus, 11, 12, 252; asexual re- 
production in, 182 

Protoplasm, 3, 24, 119; chemical 
properties of, 29; nature of, 28; 
physical properties of, 28; physi- 
ological properties of, 29 

Protoplasmic membrane, 52 

Pteridophytes, 326 

Ptomaine poisoning, 90 

Puff balls, 17, 18, 19 

Pulque, 360 

Pumpinella anisum, 354 

Pumpkin, 343; seed, germination of, 

Purity test, method of making, 131, 
132, 133 

Putrefaction of food materials, 93 


Quercus suber, 352 
Quince, 340 
Quinine, 10, 71, 356 


Rafflesia, 150 

Ragweed, 382, 383 

Rape oil, 353 

Raspberry, 340; fruit of, 282 

Raw materials: movement in plants, 
61; used by green plant, 41, 43 

Ray flowers, 169; of Jerusalem arti- 
choke, 171 

Receptacle, 160 

Recessive characters, 311 

Red: algae, 12; snow, 2 

Regeneration, 188 

Regular flowers, 278 

Reindeer: lichen, 110; moss, 19 



Reproduction, 3, 30; and wind, 238; 
asexual, 149; by fission, 182; by 
means of sex, origin of, 199; by 
spores, 182; in bread mold, 183; 
in ferns and mosses, 175; in flow- 
ering plants, 150; in plants, 149; 
sexual, 149 

Resins, 71, 352 

Respiration, 3, 30, 68 

Resurrection fern, 253 

Rhizoids, 19; of bread mold, 101, 102; 
of ferns, 176; of moss, 179 

Rhizomes, 62, 188 

Rhizopus nigricans, 101, 102, 104; 
life cycle of, 103 

Rice, 334, 335, 337, 360 

Ricinus communis, 353, 358 

Rockweed, 12, 13 

Root: crops, 47; cuttings, 189, 191; 
destruction of by transplanting, 49; 
duration of, 50; growth and temper- 
ature, 212; hairs, 48; in lengthwise 
section, 48; of sugar beet, 36; of 
wheat seedling, 50; pruning, 45; re- 
lation to soil particles, 49; storage 
of sweet potato, 46; structure, 48, 
50; tip, 120; tubercles, 97 

Root cap, 48 

Roots: adventitious, 47; field study of, 
44; factors which influence growth 
and character of, 45; growth in 
length of, 143; how they grow, 143; 
kinds of, 43; kinds of and func- 
tions, 47; primary and secondary, 
47; prop, 47; systems, extent of, 
46, 225 

Rootstock, 188, 364 

Rosaceae, 166 

Rose type of flower, 166 

Rosette, 216, 248 

Rosin, 352 

Rotation of crops, 370 

Rubber, 354 

Runners, 185; of strawberry, 188; 
propagation by, 193 

Rust fungi: diseases due to, 106; 
germinating spores of, 106; of 
apple, 108; of wheat, aeciospores 
of, 107; teliospores of, 107 

Rusts, 16 

Rutabaga, 343, 347 

Rye, 335 


Sabina virginiana, 353 

Sac fungi, 110 

Saccharum offitinarium, 344 

Sago palm, 296 

Samara, 284, 286 

Sanitation, 376 

Sap, 25, 37 

Saprophytes, 14, 83, 86; how they 
secure food, 86; how they spread, 
86; in the soil, 94; nutritive rela- 
tions of, 88; use of in preparation 
of food, 88 

Sap wood, 140 

Sargasso Sea, 1 2 

Sargassum, 12 

Scarifying, 126 

Schimper, 1 

Schultze, Max, 28 

Scientific names, 329 

Scion grafting, 196 

Scouring rush, 20, 21 

Sea mosses, 19 

Sea weeds, 12 

Secondary roots, 47 

Secretions, 71 

Seed, 147, 155; depth of planting, 
128; disease-free, 375; dispersal of 
weed, 365; fern, fossil of, 294; 
germination and temperature, 212; 
germinator, 134; nature of, 121; 
plant, life cycle of, 164; plant 
body, 23; plants, 9, 21, 327; 
plants, diseases caused by para- 
sitic, 372 

Seedling: of corn, 46; of wheat, 50 

Seeds: conditions affecting vitality 
of, 131; delay in germination of 



133; dispersal of, 279; germina- 
tion of, 124, 126, 127; hard, 126; 
how dispersed, 287; impure com- 
mercial, 365; means of dispersal of, 
285; of bluegrass, germination of, 

Seeds and fruits, how they grow, 

Segregation, 310 

Selaginella, 253 

Selection, mass, 302 

Self: pollination, 271, 277; sterility, 

Semi-permeable membranes, 52 

Sepals, 150, 151, 162 

Separation, propagation by, 186 

Sex in plants, origin of, 199 

Sexual reproduction, 149 

Shaggy-mane mushroom, 84 

Shield budding, 197 

Shrubs, 325 

Sieve tubes, 63, 139 

Silique, 283 

Simple leaf, 58 

Sisal, 348 

Smut: fungi, plant diseases due to, 
109; of corn, 17 

Smuts, 17 

Smyrna fig, pollination of, 276 

Soft wood, 142 

Soil: air of, 229; factors influencing 
temperature of, 227; fertile and 
infertile, 231; fertility, bacteria and 
relation to, 232; kinds of, 223; 
living organisms in, 232; mineral 
nutrients of, 229; physical proper- 
ties of, 223; relation of plants to, 
222; saprophytes in, 94; solution, 
concentration of, 226; structure of, 
223; texture of, 223; temperature 
of, 226 

Sofa max, 360 

Solanum jamesii, 317 

Solar energy, 6, 39 

Solomon seal, rhizome of, 62 

Solute, 52 

Solvent, 52 

Sorghums, 335 

Sorus of fern, 175, 176, 178 

Sour cherry, flower of, 168 

Soursop, 341 

Soy bean, 360 

Spanish moss, 19, 250 

Spawn of mushroom, 18 

Species, 324 

Sperm: nuclei, 146, 153; nucleus, 165 

Spermatophytes, 327, 329 

Sphagnum, 261 

Spices, 345 

Spike of wheat, 22 

Spikelet, 165; of oats, 164 

Spinach, 343 

Spireme, 302 

Spirogyra, 13 

Spongy parenchyma, 60 

Sporangia: of bread mold, 102, 183; 

of ferns, 175; of moss, 179 
Sporangium of fern, 176 
Spore case of moss, 19 
Spores: of moss, 179; reproduction 

by, 182 
Sporidia, 106 
Sporophyte: of fern, 176; of moss, 


Spring wood, 139, 142 
Squash, 343: living protoplasm in 

cells of, 29 
Stag-horn fern, 21 
Stalk of leaf, 57 
Stamen, 150 
Stamens, 151, 162 
Staminate flowers, 163; of asparagus, 

162; of cotton wood, 168 
Standard, 167 
Starch, 34; digestion, 79; first visible 

product of photosynthesis, 67; 

formation in leaves, 67; grains, 27; 

storing cells, 27; test for, 40, 77 
Stem: cross-section of six-year-old 

woody, 140; cuttings, 189, 190, 



191; growth in diameter of, 64; 

of pine, 139; section of, showing 

shedding leaf, 136; structure of 

corn, 63; structure of woody, 63 
Stem rust: life cycle of, 106; of 

wheat, 106 
Stems: growth in diameter of, 138; 

how they grow in length, 135; 

structure of, 62; structure of 

woody, 138; types of, 61, 62 
Stigma, 160, 151 
Stimuli, kinds of, 245 
Stipules of leaf, 57 
Stock, 196 
Stolons: of bread mold, 102; of 

strawberry, 188; propagation by, 


Stoma, 208 
Stomata, 59 
Stone cells, 27 
Stone fruits, 281 
Storage: cells, 26; of foods, 76 
Stored foods in seeds, digestion of, 126 
Strawberry, 340; hermaphroditic 

flowers of, 173; propagation by 

means of runners, 188 
Structure and function, relation of, 


Strychnine, 358 
Strychnos nux-vomica, 358 
Style, 160, 151 
Succession, plant, 265 
Suckering, 194 
Suckers, propagation by, 193 
Sucrose, 35, 67, 70 
Sudan grass, 347 
Sugar: beet, 344, 359; beet root, 36; 

cane, 344, 360; plants, 344; test 

for, 41 
Sugars, 35 
Sulphates, 42 

Sulphur, 42; r61e of in plants, 73 
Summer wood, 139, 141, 142 
Sundew, 250 
Sunflower, mutation in, 316 

Swamp plants, 261, 269 

Sweet potato, 343, 361; storage root 

of, 46 

Sweet turnip, 347 
Syconium, 282 
Symbiosis, 96 
Synechocystis, 11 

Tannin, 10, 71 

Tap root system, 43; of sugar beet, 

Taraktogenos kurzii, 357 

Tassel of corn, 163 

Tea, 344 

Teliospore, 106, 107; of stem rust, 

Temperature: air, 227; and germi- 
nation, 122, 123, 212; and root 
growth, 212; cardinal, 211; its 
effect upon growth, 211; of the 
soil water, 226; relation to plant 
life, 210 

Temperatures, resistance of plants to 
low, 212 

Terminal bud, 135 

Thallophyta, 10 

Thallophytes, 326 

Thallus plants, 10, 326 

Thea sinensis, 344 

Thein, 71 

Theobroma cacao, 344, 353 

Thygmotropism, 247 

Thyme oil, 354 

Thymus vulgaris, 354 

Tillandsia, 19, 250 

Timothy, 347; in bloom, 159 

Tissues: and cells, different kinds, 
27, 31; and organs, 30 

Toadstools, 17, 380 

Tolmiea, vegetative reproduction of, 

Tomato, 340, 343, 361; fruit of, 281 

Tooth fungi, 17, 18 

Toxins, 87 



Tracheid, 27, 142 

Tradescantia, 59 

Transfer of stored foods in seeds, 126 

Transpiration, 254; stream, 207 

Tree surgery, 378 

Trees, 325; determining age of, 142 

Tricholoma, longitudinal section of, 


Trifolium, 347 
Triticum, 329 
Tropisms, 247 
True mosses, 19 
Trypsin, 79 

Tube: nucleus, 146, 153; pollen, 146 
Tuber, 62, 187; of Irish potato, 36 
Tubercles, root, 97 
Turgidity, 53 
Turnip, 343 
Turpentine tree, 353 
Twig, characteristics, 134 


Ulothrix, 13; origin of sex in, 200; 
reproduction in, 201; spores of, 200 
Ulm, 13 

Umbel of leek, 164 
Unit characters, 309 
Uredinia, 107 
Urediniospore, 107 

Vaccination, 98 

Vacuole, 119 

Vanilla planifolia, 346 

Variation: heritable, 313; in plants, 

Vascular bundles, 63; structure of, 63; 

rays of, 63, 139, 140 
Vegetable gums, 353 
Vegetables, 342 

Vegetative propagation, 184, 185 
Veins of leaf, 57, 59, 60, 65 
Velamen, 250 
Venation of leaves, 57 
Venus' flytrap, 250, 251 

Vessels, 63; different kinds of, 27 
Vitality of seeds, conditions affecting, 

Vitis labrusca, 339 


Walking fern, 182 

Walnut, 342 

Wandering Jew, 59 

Water: agent in weed dissemination, 
365; amount lost by plant, 209; 
amount of in plants, 205; and ger- 
mination, 121; bacteria in, 99; 
conditions influencing intake of 
from soil, 224; content of plant 
parts, determination of, 206; dis- 
persal of fruits and seeds by, 285; 
importance in plant life, 204; in 
the soil, 224; limiting factor in 
plant growth, 205; loss in plants, 
253; measuring amount lost by 
leaves, 209; movement of in stems, 
65; power of soils to deliver, 225; 
problem of the plant, 206; require- 
ment of plant, 210; role of in plant, 
72; supply, how plants are related 
by structure to, 249 

Water cress, 343 

Water hemlock, 381, 382 

Water lily, 260 

Watermelon, 340 

Water plants, 5 

Weeds, 361; and dockage, 363; an- 
nual, 366; biennial, 366; classes of, 
366; dissemination of, 365; insects 
and fungus pests harbored by, 362; 
losses caused by, 362; methods of 
control, 4, 369; perennial, 76, 368; 
seed dispersal of, 365; seed pro- 
duction of, 364; seeds, dormancy 
of, 364 

Weeping birch, 313 

Wheat, 334; economic types of, 335; 
flower of, 165; nominating stages 
of, 123; grain, microscopic section 



of, 36; plant, 22; seedling, 50; 
stem rust of, 106 

White ash, fruit of, 286 

White pepper, 345 

White snakeroot, 380 

Whorled milkweed, 381 

Wilting of plants, 206 

Wind: agent in seed dissemination, 
365; and reproduction, 238; dis- 
persal of fruits and seeds by, 286; 
pollination, 274; timber, 237 

Wings, 167 

Wintergreen, 346; oil, 354 

Wood, 63, 139, 351; fibers, 63; hard, 
142; of pine, 141; parenchyma, 63 ; 
petrified, 294; soft, 142; spring, 
142; summer, 142 

Woody stems: structure of, 63; two 
year-old structure of, 140 


Xanthophyll, 28 
Xerophytes, 257, 259 
Xylem of stems, 63 

Yeast: cells, nature of, 89; effect of 
on sugar solution, 89; use in bread- 

Yucca, 264; pollination of, 240, 277 

Zamia, 297 

Zea mays, 329, 353; pistillate inflor- 
escence of, 166 
Zinziber officinale, 346 
Zygadenus, 381 
Zygospore of bread mold, 102 
Zygote, 306; of bread mold, 103